One of our students learned probably the most expensive lesson we've ever seen--he screwed up, and he lost a finger.
Some of our classrooms use folding chairs, and some of the students like to sit back in their chairs on the back two legs. One such student combined the two, with his hands just under the seat--near the sliding mechanism--and the chair gave way, removing about 1/4 inch of his middle finger. I got to be the administrator who responded to the situation, and I gotta tell ya, THAT was probably the weirdest thing I've ever seen or dealt with so far.
So you keep your chair on the floor, young man, or you'll lose a finger.