Similar to the Cubicle Coma (See Cubicle Coma) It takes it's toll in the same manner, but you tend to wake up either:
1) directly on the ring of the bell for the end of the period.
2) in time for lunch, or
3) when school ends.
I've had classroom coma even from early childhood. I mainly say this because I was being given a speech by the principal, only to succumb to it, fall asleep, and wake up with out a clue what happened but suddenly ready for anything.