how many collectibles are there? and is there a topless mode?
16 pickups which induce changes, 19 in total.
There is both a “topless” and “bottomless” mode (not naked, just underwear). You can unlock them by killing all ghosts in the entire school (except ones spawned by the boss - don’t worry about them) for topless; and by completing the game without taking any damage for bottomless. The options are then available in the bathroom change menus on your next playthrough.