I always carry a small knife with me when I'm flying, but that's mostly because of my survival training. Also, we usually didn't fly with a survival kit in the T-37 so that knife was my only bit of equipment I'd have after an ejection, beyond some flares and the minimalist kit (mirror, a bit of wire, etc) they put in the parachute harness.
It's not big enough to be intimidating if I tried to use it in self defense, and it isn't even particularly sharp anymore. I'd buy another one but gerber discontinued the model and the replacement designs are either smaller or thicker, so I haven't found a good replacement to buy.
I occasionally took a small pocketknife to school as a kid, but it just wasn't a big deal. I'm pretty sure lots of people had weapons on campus but nobody ever got shot, stabbed, or even beaten with sticks. The only fights were with fists and feet, and I don't remember anyone getting killed in fights off campus either. The only guy who got busted was the one selling fireworks at school... He had enough stuff in his backpack to blow up a row of lockers so the school got a bit upset when they found his stash. Even then, it didn't make the news and he only got a few days suspension. Nowadays he'd have the feds pressing terrorism charges.
Makes you wonder if the people making those rules aren't just stupid, but might actually be mentally retarded or have some sort of mental disorder. A 14 year old selling fireworks at school just isn't cause for great alarm, but you can bet people would call the FBI and anti-terrorist folks and they'd over-react in a big way.