It's possible that when you started to turn around, they saw that, realised you were afraid of them and decided to take advantage.
The best thing to do seems to be to have an Attitude. With a capital 'A'. If you walk as if you're not afraid of anything, people tend to pick up on it and respect you more. Not always possible to pull off, but if you can, it should help in most situations.
Don't show anything expensive. Walking around with a PSP, Ipod, or some other expensive device will make theives want to target you.
If you're learning Judo, keep that in mind. Same goes for being in the Cadets. The important thing is to be proud of who you are, and what you can do. Even if in a particular situation, you've little chance of winning a fight - don't let the insects who are trying to bully you see you thinking that. It comes back to what I said above, about Attitude - if you look comfortable with your situation and are confident in yourself, it'll make others think twice about trying to intimidate you. They can't get a kick out of scaring you if you're refusing to be scared.
As for your parents... it's called scapegoating. Some parents do it, and it's a horrible thing to happen to someone. Just be logical and polite to them, and the rest is their problem. I realise parents can be hard to put up with, mine are chronically stupid a lot of the time - hang in there, though. You'll only have to live there a while more and then you'll have your own place, and will be able to decide how long they can visit you for.