Find a high school that does this already. You can see what they did to get the radio station. You can check with the closest Community College, they will have their own radio station, you maybe able to utilize their resources and the school and district wont need to buy the equipment.
Talk to your principal. and see what they say. Circulate a petition, show a demand. Go to your school district school board and talk to the board members, someone will like your idea and give some help.
The equipment to broadcast can be pricey, not to mention the hassle and red-tape to broadcast. So your best bet maybe the Community college involvement or one of the many AM stations in your town.
Best thing is to start asking questions, and talk to each organization. It will take a lot of foot work so be sure you are committed, people will see that and want to get involved.
Good Luck.
For those who wish to start a campus-limited radio station you can find some excellent information and resources over at CampusBroadcaster.net.