I'm STILL stuck on the potion quest too. Waiting on that walkthrough I guess, the both of us.
Having an in-game hint system is great but I feel like I've just straight overlooked something that isn't a quest and that's why I haven't seen like 4/5ths of the content in the game.
You need four ingredients: moon mirror, four-leaved clover, pamela flower and owl's feather
You can get the Moon Mirror from Vanessa in the library, then activate it on Saturday or Sunday night.
You can get the four-leaf clover from Catherine in the infirmary.
You can get the pamela flower from Berelman (the raccoon) by fulfilling his request to get the socks.
You can get the feather of an owl from Berelman (raccoon) if you ask him to distract the director, for this he will take 10 gold coins from you, then in the morning take the feather in the room of the director. Only before that you need to ask the director in his room about the ingredients.
After you have collected all the ingredients give all the items to Vanessa in the lecture hall. Turn the time to evening in the lecture hall, then the minigame "Match 3" will start. Finish the minigame and go to bed in your room.