There are door jamming devices that would require 1,000slbs of force to break, and with a solid wood door it would be impossible to breech. You can also use a fire extinguisher as a weapon. Spray it in the person’s face and then bash them over the head. A teacher could keep a can of bear spray in every class room. A letter opener or pair of scissors make a great stabbing weapon. Basically anything can become a weapon with a little imagination.
How about just a gun?