Okay, if you're at the point where Serena is ready to challenge the club, then you need to make sure she doesn't lose: she needs to resist hypno mindbreaking in the chair and also needs an insider to help her. For the first, I recommend buying the appropriate amulet. For the second, you need to talk to the member the club has brainwashed and persuade them to help.
More info on step 2: The brainwashed club member left Serena the note that starts the ball rolling on Serena remembering what's been done to her under hypnosis. The second note looks like a love letter on Serena's desk in the classroom. If the note is missing, look behind the utility shed, one screen left of the school's main entrance. If there is a student there, talk to them wearing the basic teacher suit with trousers, no skirts (the brainwashing prevents the student from interacting with provocative outfits).
That should get you to the Good End!
The door next to the staff room is relevant later in the game, progress the main plot for now. If memory serves, it's usable in the Fantasy/Alternative School. I'm hazy on when you access that, but I believe it to be in the endgame.