School doors should all be solid cored. Standard locks are not bullet resistant and therefore some form of door stop should be used in conjunction with door locks to help prevent forced entry into classrooms by armed assailants. Here are a couple of very simple, inexpensive door-stop designs that can be used to prevent entry into a classroom even if the door doesn't have a lock.
A simple Security Door Stop constructed from 3/4" steel that stop an intruder from entering a classroom. Fits into two holes pre-drilled into
the floor's concrete slab. High heel shoes should not be worn in schools that employ this type of door block, and one should make sure that
there aren't anywater pipes, electrical, heating or other obstructions where you plan on drilling the hole.
This is an incredibly simple $2.00 Door Stop that can be purchased from virtually any metal supply company.
It is a ten inch piece of 5/8" cold rolled steel. A 3" x 3/4" diameter hole can be drilled into a 4" thick floor slab directly behind
a closed door. If there is an active shooter situation simply close the door and drop the steel into the hole.
High heel shoes should not be worn in schools that employ this type of door block, and you want to be sure that there aren't any
water pipes, electrical, heating or other obstructions where you plan on drilling the hole.