I'm not any kind of technical support for the game or anything, but have you tried either entering a classroom, exiting again, and then trying to leave or saving at the spot to where you've rolled back, quitting, restarting, and loading that save?
I don't know what's caused the bug, but it's possible some variable didn't get reset properly or that a subroutine/function was interrupted, so the code for the option to exit didn't get executed as it should have. These are general types of issues that can happen in various games / engines, so they are just a couple of stabs in the dark for you.