The police CAN-if they choose-perform a "Stop & Frisk"...withOUT any cause, reasonable suspicion, etc. Frisking you, and finding a knife on you is all they need to arrest you. When they do, that "everybody in my school does it" line is NOT going to help you. It will just cause the cops to stop & frisk more kids from your school. When others find out that YOU are to blame for that, you will have real problems...with kids who want revenge.
Well you can carry a weapon legally, as long as it ISN'T concealed (of course this is only in most of the states).
If you want to carry a concealed weapon, you must have a CPL.
As for "brandishing a weapon," here in Washington, you can't be arrested unless you have out the weapon with intent to threaten another person's safety. If it is for a magic trick, opening stuff, cutting open packaging, etc. it's legal.
The other clause is "no-knife zones" which are mainly schools, and airports.
Simply wearing the knife on a belt clip gets around "concealed weapon" laws.
The last time I was searched (3 days ago) the officer asked if I had any weapons on my person, I said yes, and he asked me to put them on the hood of my car. He examined several of them, asked what kind of material one was made of, said "cool" several times, and returned them. All of the items where concealed - he never asked for a permit (then again, C&C permits are typically only for firearms).
Most cops don't care about hassling you if they dont think you're up to something. In my case - I actually WAS acting suspicious. None the less, the officer was nice about it.
In any case, if you're under 18, you have no business carrying a knife. I credit alot of my lack-of-hassling to looking like a business man. Secondly, I carry functional knifves and weapons - not gawdy showpieces like the one seen in the mentioned video.