I went to a small private school for highschool, the principle had the parking lot and all entrances wired for live video feed and a wall of TVs in his office, if anybody came into the premises at irregular hours he knew and would intercept them, the main entrance automatically locked and you had to be buzzed in to be let in after school started or the doors could only be opened from the inside, the only glass doors were the main entrance which led to the main office, other doors were solid construction, no breaching them, all classrooms had 2 inch thick solid wood doors with 2 locks, all windows except the cafeteria were 6 feet above ground, cafeteria windows were on the ground and the cafeteria was in a basement level. The principle had a safe in his office with an AR, 3 loaded mags, and a pistol, the vice principle kept a pistol in a safe in her office too and the receptionist had a high pressure bear mace can under her desk. The building was only made up of straight hallways no intersections, just stairs or 90 degree turns and cameras at both ends of ever hall. In short, a tiny bit of concern and planning would make such things impossible.