NFPA 101: Life Safety Code states that any door within a fire/smoke partition must be rated for the partition and must positively close. A classroom door is within a fire/smoke partition if the door is rated as a fire/smoke door (the door will have a metal tag on the hinge side of the door providing the door’s fire/smoke rating) and has a mechanical closer installed.
If this is the case, then Life Safety Code requires that the door freely close. At no time is the door allowed to be “propped” open.
If the classroom door isn’t part of a fire/smoke partition, then NFPA 730: Guide for Premises Security suggests that the door remain locked at all times.