It has to be noted that most colleges and major institutions use commercial hydraulic door closers that are easy to jimmy to make them hard to open. Next time you go to a restaurant look at the closer and notice how the two rods that are hinged together work. If you can keep those two rods together the door won't open.

A boot lace will do it. Tightly lash the two rods together near the closer and you limit access without having to lay in front of the door and risk getting hurt. Make up for any weakness in the cordage you use by using more wraps. Also, the tighter you get the lashing the more tightly the door will remain closed.

Odds are that a determined attacker will be able to kick and shove the door open a bit and, given time, they may be able to slip through the narrow opening. This tactic mostly limits their mobility and buys you time that can be used to make a strategic retreat, or organize resistance. Time and distance are your friends.

It also has to be noted that a person, even a strong person armed with a gun, is vulnerable if they are forced to slide through a narrow gap. If the door is kicked or rammed when they are half-way through the narrow gap you have a fair chance of causing serious injury.

It also has to be noted that lashing the closer can be combined with wedging the door and barricading the entry with anything handy.