Should Young Transgenders Need Documentation?

My girlfriend Alex who is transgender, was taken out of our school dance because she had a feminine shirt on and abit of makeup. The school counselor had said that my gf was being disrespectful by telling the counselor to call her mother. The principal called the outfit a costume. Alex's mother was told to "show medical proof" of Alex being trans. Transgender kids shouldn't be told that them being themselves is a costume nor being taken out of a school event for their outfit. Transgender kids in my school are told to show documentation before they are called the name and pronouns they want to be called. I want transgender kids to be able to go to school and be proud of their identities and not told their outfits are costumes.