I found the 10th language notebook in little puss mode in the green room to the right of the 9 room. I had plenty of time to reach the elevator, since I wasn't locked in.
Viewing post in Advanced Education with Viktor Strobovski comments
Yeah I found 10 and that wasn't counting the one IN the 9 room
In every run so far, there was a Language notebook in the closet (dog). So I tried skipping that one, but the door still wouldn't open until Marzia did it.
Wonder what happened if I...GO UP MR. CLEANY'S LIFT
It worked but nothing of much interest happened apart from breaking the cinematics lol
Did you know that in language, there is a classroom that can only be opened with all 3 colored keys, red, blue, and black? There are 3 overlapping doors with yellow, blue, and red handles. It leads to a useless classroom with nothing interesting except flipped and stacked desks and chairs. Hardly anything useful except a few generic items. I only checked it in little puss mode, maybe there's something more interesting in it during the chase modes. When you reach the 2nd half of the level (the area past the server room and the gross pizza room) look for a single door with a yellow handle and have all 3 colored keys with you (not counting the history level's forest key). It's one of those classrooms with only one way in and out.
Uh, well, I guess it's for Mr. Cleany-Clean to get between the levels on his own. I think the other elevator in the Math Level (the one behind the picture in the greeting room) leads to the Language Level, since instead of arriving at what I thought was the entrance door, now you arrive at the elevator.