Require all teachers go through advanced weapons training, and also require that all classrooms be equipped with a "Break In Case of Emergency Box" In the box would be ONE AR-15 Semi-Automatic rifle with 60 rounds of ammunition in two 30 round magazines and ONE 1911 style .45 with three 8 round magazines.The boxes that hold these weapons are only for gunman scenarios in schools and the boxes will have alarms on them that notify the police if they are tampered with.
Also retrofit the schools with a lock-down alarm that if triggered every door in the building (bulletproof) is magnetically released causing it to close and once closed will not open without a special key that only authorities and maybe the principal posses.This would prevent a room to room massacre