Two words to remember when you start thinking about putting locks on classroom doors:

FIRE CODES

It depends on where you are, seating capacity of the room and so forth, but local fire codes may restrict the way you can lock the door. For example, you may have to use a push open or push down handle that releases the lock.

While being able to lock the room down is a good idea, you also do not want to do anything that will prevent rapid exit from the classroom.