Consider Electro Magnets
At my workplace, we had electro magnets on all doors that once closed would not let anyone in without permission but would let anyone out with a push of a button. In case of a fire, all doors would unlock and let everyone out. It worked on all the doors at my workplace very well. Why not apply it at the schools? It is an inexpensive way to keep our children safer while at school. Check it out — if it saves one life, it will be worth it.