They should be sued.
Sued for what? Having a stupid dress code? Can I sue my place of work for not letting me wear my “No Fat Chicks” t-shirt?
Listen, it was a stupid decision, but it was the school’s decision to make. Their house, their rules. Don’t like it? Join the school board and change it, or home school.
Everybody wants to be a special snowflake now and think they are exceptions to the rules. This would be a good lesson for the kid: sometimes life isn’t fair.