The greatest tool you can edc is the grey stuff between your ears. you can improvise most of the items that are not permitted on the campus with a little thought of what is around you. I would look at adding a whistle, photon light to a key chain.
Just think of what is around you in a school. for instance take the tiny razor blade in a pencil sharpner it would work in a pinch. you can make a tool for just about anything if you put your mind to it. steel whool and a 9vt battery can make a great fire starter but each on there own would not brake any rules that i am aware of(on a school campus). The main point is i havent been in school for about eleven years know and i can still think of things that were around that i could use for a survival situation or disaster. it will almost never be as important as what you have compared to what you know. there are exceptions to that but many have died who just didnt utilize what they had for not seeing beyond the intended use of an item.
Good question and thinking ahead of what could happen.