Hey thar!
I'm 15, too! And until only about early this year i've had the same sort of problem. I still go a little red if i say anything wrong or something like that, but not immeadiately. People have already suggested all the really good ideas, so i'll just blabber on, and you can read if you choose to.
I suggest you join some sort of club or activity, not necessarily with school.
Last year March 17th, (my birthday! :lol: ) I joined something called the ATC, which is the Air Training Corps, commonly known as Air Cadets. It's good there, you immeadiately meet a whole load of new people, but because new people are arriving all the time, it's nothing like being the new kid at school. And everyone's always so friendly, and rules about bullying are far stricter and more enforced than at school.

It's improved my confidence so so so much, and i'm friends with the type of people i'd never be friends with just being at school.
If you're interested in something like this, there's the Air Cadets, Sea/ Navy Cadets, and Army Cadets. I'd definately recommend Air Cadets.
I love the discipline of it all, and it's still great fun. If you're not too keen on that sort of thing, maybe you could join guides or some sort of youth club?
Anyhue. I'm so much better now, and i can have a short conversation with people i don't know, plus, it gives you a slightly more authoritive feel over the people that would perhaps bully you. And i know that there's some sort of aura that i give off now, that makes people like that avoid me.
Anyways. Best of luck! x