Best thing to do in the case of a school shooting, bring a brick to school in your back-pack. Keep this brick close and when the shooter comes, hide in a place that won't immediatly be seen, let him enter, then throw/smack the brick at them/over their face.
Simple and easy, oh and my science teacher last year was a navy seal before, so he told me that he would rush the person and tackle them. He said that in such a situation, it's either stand around and get shot, or do something and possibly live, this guy also taught me several navy seal techniques, like how to use your elbow to smash peoples bones