my classroom was pretty typical for a public high school... outward opening steel entrance doors with probably 8x24 section of reinforced glass for vision...several inner doors leading to common office and other classrooms... during "lockdown" had a section of opaque paper to cover window...for active shooter my plan was to use wedges for the inward opening doors and had precut 8' pieces of 2x2 for outward doors...two loops of 550cord per 2x2 section, half hitch from each loop around door knob... adjust length of loop so wood is in contact with door frame diagonally crossways... pretty resistant if agressor has only outer door knob for purchase