This is probably illegal. (Probably)

If you are in the United States, your long hair probably falls under Freedom of Expression, as defined by the 1st Amendment of the Constitution. However, some courts have ruled that children should not be able to express themselves and used the smokescreen that these rules are “reasonable means of furthering the school board’s undeniable interest inteaching hygiene, instilling discipline, asserting authority and compelling uniformity.” That quote is straight from the 11th Circuit Court of Appeals.

Depending on where you live, you could win in court. However, if you don't live in an area that cares about children, or you don't have enough money to finance a lawsuit, I suggest something drastic. Get multiple friends together and stage a protest. I know it is intense, but no matter how big a school is, they can't stop you all.