Well a few questions:
Is there a lock on the class room door? There should be a double key dead bolt if there is a worry of the lock being used inappropriately. The class room door should be locked.
Is the furniture movable? If so they should be barricading themselves in side the class room that they know is safe.
Are the teachers allowed to have cell phones in the class rooms? If so the teacher should then get on the phone to 911, “Room number XXX is secure, we have barricaded ourselves inside, there is no shooter here. We will come out when the police arrive, have the situation under control, and the police are outside the door to escort us to a safe place.” Then when the police are at the door, the teacher needs to ask for the name of the officer, and communicate through the barricade to verify it is a real Law Enforcement Officer on the other side and not a ruse.
A single desk will not any protection to firearms, but 20 of them in front of the door will help prevent the bad people from coming into the class room.
It sounds like they have taken the Earthquake Drill procedure for everything.