Hey Samsurf3, you have a kinda tough question there because I think most folks here don't want to set you up for a bad time. I'll try and help you. First by intermediate you mean you can catch and ride waves fairly well and do some ok turns? That's not bad if you are there.
I'm not sure what contest you are gonna go for but it could go either way from what I've seen as far as the level of skill of your competitors. Most likely there will be a fair number of other 15 year olds that will have some pretty solid skills, but if you go into it with the idea of trying to have fun, and just give it your best shot, it could be pretty cool experience and you might like it even if you don't win a single heat. You get to watch some good surfers in the different age groups and learn from them, meet a bunch of cool people. Overall these things are usually pretty fun.
Here's an idea, maybe you go and watch one contest to get an idea of your competition? Explain your situation to your parents, tell them what you said here and hopefully they will take you to just check it out one time. If it looks like you'd be ok doing it, do another contest by the same organization so you'll probably surf against the same people you saw in the one you watched.
Good luck Samsurf3