Here are some (better?) ideas:
1) First and foremost, all entrances to the building should be secured at all times. Easy to do at a minor one-time expense
2) The door to every classroom should be bullet-proof, and locked from the inside at all times. When the bell rings, the locks are automatically disengaged long enough to allow exit/entrance to the room. The "inside" lock allows for emergency exiting. A small one-time cost
3) At least one school administrator should be required, as part of the job, to be professionally trained in the use of a handgun, and to carry the handgun at all times during school hours. There would be no "staggering cost" to taxpayers - the administrator is already being paid for their job. This would simply be a new job responsibility. There would be a small one-time cost for training.
I'm sure these ideas can be fleshed out more, but this provides some reasonable food for thought.