None of them are hidden anywhere the camera won't show. Just search each room as thoroughly as possible. Also, some of them are *extremely* difficult to safely get without using your attack, so you might wanna unlock it before going all in on the search. (If I had to guess, you either missed the one in the clock tower that's most visible from the top of the stairwell, or one or more of the ones in and around the staff room)
EDIT: I may put together a complete location guide later if I can.