Every year, the ACLU receives calls from students whose schools have told them that they cannot bring a same-sex date to the prom or homecoming, or that they can't come to a school dance unless they wear clothing that conforms to traditional gender norms. Policies such as these exclude LGBT students from participating fully in school life, and they're not only prejudicial, they are unconstitutional.
In Fricke v. Lynch, a federal court ruled that any policy excluding same-sex couples from proms or school dances violates the right to free expression guaranteed by the 1st Amendment to the U.S. Constitution. Other decisions have found that enforcing outdated notions that only boys can wear tuxedoes and only girls can wear dresses to formal school dances is illegal. Other federal courts have ruled in the same way, most recently in the case of McMillen v. Itawamba County School District.
The ACLU's LGBT and HIV Project has written a letter to principals and superintendents that students can use if their school tries to stop them from bringing a same-sex date to the prom or any other school dance. We've also written a letter to schools about the right of students to wear clothing, both at school events and every regular school day, that may not correspond with their sex assigned at birth. You can print whichever letter is appropriate for your situation, give it to your principal, and ask that they please change their position on school dances.