I'm a high school teacher of 13 years, a department chair, and member of our school's leadership team which entails emergency planning and policy among other duties. We regularly carry out lockdown drills to simulate a shooter or other threat on campus.
People in past posts have mentioned security and a school's duty to keep its students safe.
As far as my school is concerned, and I believe my school as an accurate representation of the situation at most schools, the only reason we have not had a violent incident, from a security standpoint, is luck.
If you can jump an 6-8 foot fence, you can be on a school campus in seconds. But that isn't necessary; given the crowds, multiple entrances, etc., a random person can walk onto a campus during morning drop-off with ease. A parent or other "known" face can generally enter without question during morning drop-off.
Sound like we need more security? Talk to your local district about funds. My school has 2400 students and precisely 4 security guards, none of which are armed. We have one police SRO (special resource officer), but he rotates amongst various campuses.
Unless people want us to begin turning our schools into facilities that more resemble prisons (secure entry checkpoints, metal detectors, locked doors, etc.), we are safe by luck alone.
Think about the world we've created here.
I guess the fact that we regularly conduct drills and plan for a "shooter on campus" by locking doors, drawing blinds, and having minors crawl under desks away from windows is a direct by-product of our Freedom to have 300 million guns accessible by just about anyone that wants one, including kids.