If it was my daughter, I’d sue the school district for every penny I could get out of them.
The school was following the law. They were compelled to allow the boy to play. I don't think you can successfully sue someone for obeying the law.
I believe that's standard procedure. The school wouldn't allow an underage girl to play without parental permission and a waiver of the right to sue.
The leftists would target the father. Her school would too. They would lose.