If you are getting harassed at school for being gay, chances are this isn't going to seem like a safe environment where you will find support. This can be especially true if teachers ignore what is going on.
For some teens, however, school can be a great place to find help. If your school has a GSA you can start there.
Also, some teachers and guidance counselors offer what they call "safe zones" for GLBT students. Many display a gay pride rainbow, or a similar logo with the words, "Safe Zone." If you see a sign like this, it is an indication that person identifies as a GLBT ally.
But even if you don't have a "Safe Zone" program (and most schools don't), that doesn't mean that there are going to be grown-ups you can turn to. If there are any openly gay teachers or administrators, you can start there, but plenty of straight adults can offer support as well. A good person to talk to is your school nurse or guidance counsellor.
And if you meet resistance, you can seek advice from a group like the Gay Straight Educational Alliance (GLSEN). Additionally, if you think your school infringing on your leal rights (say by telling you that you are not allowed to bring a same sex date to prom, or by censoring GLBT-themed websites), you might want to consider contacting the ACLU, a public interest law firm which has taken on many school cases for GLBT teens.