I carried a knife through high school and now in college. I never got in trouble for it because I didn't go around showing it off, just used it when I needed it and put it back in the pocket. It also helps that I'm an engineering major. Even at work, which is an elementary school that technically only allows a 2.5" non-locker for staff EDC, but they do have a wobbler for other things that are reasonably related to your job. I coach our robotics team in the spring, and I'm generally known around campus as "Mr. Boyandhisdogs-the-science-guy" (cue the Bill Nye music) so if anyone gets a pass to carry a leatherman, it would be me.
I live in a very militantly liberal part of California too. The trick is that I'm a good kid who stays out of trouble and minds my own business, so I don't attract attention to myself in the first place. I bet I could get away with just about anything, but choose to follow the law on the honor system... well mostly. I'm also scared of that .0001% chance that I'll bump into one stickler of a cop and he'll really ruin my day.
As someone who works at an elementary school, let me tell you that most kids I know, even the 4th and 5th graders aren't mature enough for a knife. I know kids that age CAN be capable of responsible knife ownership, I got my first pocket knife at 8, but the environment these poor kids are being raised in is what's at fault here. Even if one kid is responsible enough to carry a knife, there's a decent chance that his buddies are not. Being a kid, he'll show it off too his buddies at some point, and all it takes is one knucklehead to snatch it and run around with it to ruin it for everyone either by getting hurt or getting caught.
People around here would rather wrap their kids in bubble wrap and stick their heads in the sand than try and teach them personal responsibility. So they're taught to be afraid of and avoid everything "dangerous" like knives, guns, tools, etc and never get taught how to handle such things safely. Well one day their curiosity is going to get the best of them. Not to mention the fact that by only seeing pain/death/injury in movies and video games, they don't take it seriously. They play and joke about killing people and treat knives/scissors/other sharp tools like toys because they just flat out don't understand the consequences and gravity of what they're doing. I know this sounds harsh but until you witness an untimely death (or better yet, cause it yourself) you don't truly grasp how real and permanent it is.