I hate to be the voice of dissent, but I know a great deal about wigs and I can enlighten you a bit here. First of all, the very best way to have everyone who looks at you say "there's a crossdresser" is to wear a cheap wig. Cheap wigs tend to look cheap and phony. About the best you can do is get a very dark one in a simple style and try to not to get too close to anyone.
If I were you I would reconsider some way to grow my hair out some. A lot of women wear shorter styles that can easily be transformed to a more manly or womanly look. Look at photos online or talk to a stylist.
I don't know just where you live, but in much of the world, longer hairstyles on males seems to be acceptable in schools these days. You might have to deal with a grumbly administrator or two or your parents, but having long hair is a wonderful experience that shouldn't be denied to any young person who wishes it. And trust me, you want to grow it out now while you're young and still have plenty of it.
