The answer to this question is tough in that it really does depend on the individual situation. In general, non-rated classroom doors may be propped open but only with an approved hold-open device.
Under no circumstance would it be acceptable to prop open a classroom door with a kick-down device, a wooden wedge, or a trash can. Examples of approved hold-open devices include but are not limited to: magnetic hold-open devices or friction devices. Any hold-open device installed on a door must only require 1-motion to close.