If you are in high school, there are a lot of programs where you can find a host family. You don't have to pay for school, but you will have to pay for travel and your own expenses (spending money, going out, clothes ect.) There are different companies and organizations who will set this up for you, however you do have to pay a fee in most cases.
Even if you find a host family, you can't just come over and start going to school, since that is illegal and you could potentially get deported. The companies take care of all the legal stuff for you as well.