Simple, Cheap, Partial Protection for Classrooms.

This is not perfect, does not offer protection for first classroom attacked, but does offer a lot for other classrooms, and does offer protection for all if the intruder is detected before he fires a shot.

Basically, push a filing cabinet, stationed near each doorway, into the doorway, and chain it in place.

On each door, place two 1/2 inch eyebolts, about 3 feet up, eyebolts protruding into the classroom side.  Attach enough chain to each eyebolt to reach around one or two filing cabinets, and then be locked in the middle of the classroom side, if an intruder event occurs. Normally the file cabinets are next to the doorway, not blocking the doorways.

These file cabinet are preferably 5 drawer legal size.  The top drawers can be filled whatever stops bullets the best.  Could be old textbooks.  The bottom drawers are available for storage.

A shooter now has to contend not only with shooting a door's lock off to get inside for a range of fire, he has to deal with filing cabinet(s) to get inside. He is also somewhat blocked from being able to spray the room while standing outside.  A steel plate extending upwards, attached to the side of a filing cabinet, would completely block his view.

Used filing cabinets are cheap, typically $10 to $30, at Habitat Restore, or though the County Surplus sales.

Installation can be done by parents, and the bolts and nut protruding to the outside can be covered by a 2 x 6, which in turn can have a steel plate for further protection and concealment  of bolt placement.  Let the shooter expend his ammo and perhaps even injure himself, trying to get through.  Perhaps local hardware stores could give discounts for eyebolts, chains, and locks.  Total cost should be under $75 per classroom door.

Teachers and students working together can push cabinets in place.  Every school should have it's own "safe word" to sound the "all clear, you can unchain the cabinet(s)."

No Second Amendment rights are harmed by this procedure.