The game can be fairly non-linear at points, and some events are entirely independent of others, so it is fairly difficult to really make a walkthrough to cover everything. I am going to try my best to cover the paths that you can end up locking yourself out of though. There will be a lot of spoilers ahead.
Do note that this walkthrough is for 0.17, and may not be complete for later versions.
This plotline is revolves around the Main Character attempting to use their powers for good. As you are not drawing attention to yourself by building a Harem, Mr. Drape will begin his plan to turn the school's student body into his idea of perfect students, starting with your friend, Val.
To get on this plot line, you will need to avoid making any character your slave. This does not mean you cannot use your magic on other students, strictly speaking, but you will need to avoid outright enslaving anyone.
In the tutorial, most of your choices just influence Val's opinion of you. The major decisions you need to watch out for are after obtaining your magic; you can choose to test your magic on her, or on a monster. Using your magic on Val will subsequently lead to a choice on whether or not to keep her under your control. Choosing to keep her will lock you out of this plot line!
Once you return from the Dungeon, you will be free to roam the school and trigger most scenes in any order. You should visit the Mr. Drape's room to get Sarah to join your party, and Jane's room to get permission to enter the dungeon. You can enter Dr. Tiff's room at any time, but you might as well do it now to get it out of the way for later.
If you talk to Val in the hallway, you have a chance to enslave her, but that will lock you out of the plotline. Trying to enslave her, and failing will instead have Val leave your party but will not lock you out.
The next major plot point of this plot line starts when you have reached the Fourth Floor of the dungeon. At this point, a new conversation with Val will become available. She is now dressed almost exactly like Mr. Drape, and insists that it was her idea. The next conversation takes place in Dr. Tiff's room, where you can report Val's condition to her.
Jane will then meet you in the hallway outside of Dr. Tiff's room appearing to know a little too much about Val's condition. She offers to help, and will wait for you in her room. After accepting Jane's offer, you will need to make sure Sarah is in your active party, and visit Val in Dr. Tiff's room in order to enter Val's mind.
The first floor of Val's mind is meant to vaguely resemble a roller skating rink, and is populated by sentient Roller Blades. Find the pulsating Sigil to go to the next floor. The second floor vaguely resembles a diner, and if you attempted and failed to enslave Val a new enemy resembling a broken heart will appear in battles now. Find the pulsating Sigil to reach the inner-most part of Val's mind and fight the boss guarding it.
Once you have defeated the boss, you will be instructed to cast a long-lasting spell in order to bind Val's soul to your magic, and prevent Mr. Drape's magic from being able to bind to her again. You have several choices at this point:
Suppression: This will enslave Val, and result in your path changing from the Heroic path to the Tempted Hero path. (No scenes have been made for this path yet)
Confusion: This will permanently bimbofy Val. She will not count as your slave, so you can continue the heroic path if you cast this spell.
Grapple: This will permanently turn Val into her Goth personality, and give her a sudden, new interest in bondage. She will not count as your slave, so you can continue the heroic path if you cast this spell. (Discovered a bug with this spell after already starting on the next update. It will work properly in 0.17)
Charm: This will amplify Val's feelings for the main character, and make her permanently in love with your character, regardless of your other romantic interests. She will not count as your slave here either, so you can continue the heroic path if you cast this spell.
Boost: Boost can improve the body in any way, which the main character normally uses for healing. If used here, you will make an imperceivable change to Val's body in order to bind her soul without side-effects.
Once you have left Val's mind, you need to enter Dr. Tiff's room one more time to check Val out from her care. You can put Val back into your party in your room after this scene concludes, and this is the end of the heroic path for now.
Faculty Control Path:
The first thing you need for this path is to enslave Val. You can do this by choosing to test your magic on her, continue casting on her after she warns you to stop, and then keeping her. Alternatively, you can enslave her when she talks to you at the school by casting Confusion, then Suppression on her.
If you enslave Sarah prior to reaching the first note, you can also Enslave Val by having Sarah in her "Eager to Please" form when you touch the note. In the cutscene that follows, you can cast Confusion, then Suppression on her to break her will.
Go into the dungeon, and explore a bit. Find the note on the Third Floor, and attend the first Secret Class with Ms. Rack. Once you have finished, you can enter the library to the right of the dungeon to meet Julia. Do not enslave her. You can make any choices in this conversation you like; you will either agree to help her, or leave the possibility of helping her open.
Once Val has reached lvl 4, you can return to the school and enter Ms. Rack's room where there will be a scene in which she will ask you about Val's odd behavior. You can tell her the truth, or lie to her. This decision will have an impact on her opinion of you, but neither choice will prevent you from enslaving her.
If you have not already, your next target should be to find the second note, located in the top left room of the Fifth Floor. Julia will not allow you to play with her yet, but after giving this note to her, she will allow you to play with your other slaves, and she will tell you Ms. Rack's weakness.
When you return to your room, talking to Val will give you a choice on whether to give her the plan, or wait. If you have not yet made her into the personality you want her to be during the next scene, this is your last chance to change her. When you tell her the plan, the next time you talk to her will trigger the scene with Ms. Rack.
There are a couple ways to fail this scene and end up Ms. Rack's slave. If you want to avoid them; or deliberately seek them out they are:
1: Have Val be in her normal personality and tell Ms. Rack to have a make-out fight with her. Val's hesitance to make out with her Aunt will lead to her failing the "battle" and Ms. Rack turning her against you.
2: Have Val be in her Bimbo personality, and tell Ms. Rack to have a make-out fight with you. Val's bimbo form is unreliable, and she ends up spiking both of your drinks, causing you to be too tipsy to resist Ms. Rack's power.
Student Control Path
There is very little to this path so far. It begins when you have enslaved at least 2 characters, and have not started the Heroic Path. When you visit Jane, she will confront you about your use of your powers on other students.
She is not angry with you, however. For some reason, she wants you to keep using your powers; just a little more discreetly. She even offers to help you hone your powers for purposes that the stuck-up Mr. Drape would never allow.
1: During your first trip into the dungeon, if you test your magic on her, and keep going at every prompting she will become your slave.
2: During your first random conversation with Val in the school, you can cast Confusion on her, then Suppression to enslave her. If you do not cast Confusion, she will leave your party.
3: If you have Sarah enslaved and in her Needy form, and both Sarah and Val in your party at the time, Val will notice and question her behavior. You can enslave Val by casting confusion, then suppression in this conversation.
In order to meet Sarah, you need to attend Mr. Drape's first class.
1: After attending Mr. Drape's class, and Dr. Tiff's class, Sarah will appear in the hallway, still affected by Mr. Drape's curse. She's twisted her ankle trying to walk without using her arms for balance, and needs someone to help her out. You can either use suppression at any time to force her under your control, or you can offer to heal her, then cast suppression instead of boost for a different scene.
2: If you refuse to heal Sarah in the hallway, she will come after you on the streets. During this conversation, if you have enslaved Val already, you can enslave her at any time, or you can goad her into attacking you first for a different scene. If Val is not your slave, she will stop you from enslaving her.
3: If you have Val enslaved and in her slave form, and both Sarah and Val in your party at the time, Sarah questions Val's behavior. You can choose to either admit to putting Val under your control which will cause Sarah to challenge you to enslave her to prove that you are lying, or you can cast confusion, then suppression on her at any time.
1: After you have attended your first secret class, Julia appears in the library. When talking to her for the first time, at any time you can use the confusion spell on her, followed by Suppression to make her your slave.
See faculty control path
In order to meet Cassandra, you need to upset her. You can accomplish this by either being friends with free-willed Sarah, and standing up for her during one of her conversations when Cassandra shows up to make fun of her, or by making Julia into a fanatic and agreeing with her when she makes the claim that you are stronger than Cassandra.
1: Cassandra will challenge you to meet her in the Dungeon for a duel the first time you talk alone. When you go there, she will offer to join your party if you pretend that you lost. Refuse the offer and duel her. During the duel, cast Boost on her instead of attacking her.
After casting Boost a few times, the battle will end, and you will get to choose what kind of cow you prefer her to be in her cow form. Cassandra will demand to be changed back immediately, and you are given the option to leave her as she is, or return her to normal. Choose to return her to normal, then cast Suppression instead to enslave her.
There are several scenes that will play out at random in your bedroom if you have the right conditions met. These are the conditions:
1: You need to have enslaved Cassandra and have her in her Cow form, and you need to have enslaved Sarah and have her in her Tough form.
2: You need to have Val, Sarah, and Cassandra enslaved, and all in their normal personalities.
3: You need to have enslaved Cassandra and have her in her Maid form, and you need to have enslaved Val and have her in her Bimbo form.
4: You need to have Sarah enslaved and in her Needy form, and you need to have Julia enslaved and in her Fanatic form.
5: You need to have Cassandra and Julia enslaved, and both in their Normal forms.