Here is what I would support.
Metal detectors in all schools. If the states can't afford them, have fun raisers. Allow each community to raise their own funds! I think with the way things are today, mostly everyone would contribute something. I hear they're costly, but I don't know how costly.
Get police inside the school entrance at the beginning of the day as students go thru these detectors. They don't have to stay all day, but just as school starts. I know our town would do this. Heck, we already have upteen cops circling the school grounds at the beginning of the day. Get em out of the vehicles and into the school lobby.
Once everyone is in for the day. Lock the school. That means ALL doors. Get the buzzer system that most schools have today. Once in, find out what the person wants. Don't just allow them to walk freely thru the building. Have them sign in and out and also go thru the detectors.
2 other things I can think of real quick. If your school is also the public library (I recently read this happens in some towns and was shocked), only allow those in the town into the library after school hours or get rid of these types of programs altogether. I'd also suggest for schools that are so large that students must go outside to change classes, do away with that. Give students a bit more time, but mandate they stay within the school.
IMO, none of these things are so drastic that they're too much to deal with.