All students do not have to participate. Don't let your son go to school that day. Grow a spine. Write to the superintendant. Write to the principal. Write to the teacher. Let the PTO know about this.
Or, let your son go to school dressed as normal and go as a tomboy. (smile)
Or, let your son go to school dressed as normal and go as a tomboy. (smile))
And then see a judge about a restraining order on any adult facilitating this, enjoining them from coming within 1000 yards of a minor