Ive taught a few young children (primary) who have worn clothes aimed at the opposite gender/sex.
Girls who have worn clothes from the boy's sections and girl who comes to school in a school dress. Some have been more insistent over it than others in terms of being perceived as the opposite sex. Usually they just get on with it.
Without exception the children in the classes - up to Y2 - accept this with no issue whatsoever. Some, children and adults, occasionally get mixed up over if that child is a girl or boy, but it isn't ever a problem. It's just a boy wearing a dress with long hair tied up, or a girl with cropped hair and 'boy's' design clothes.
If any parents have an issue - and we are in a VERY multi cultural area and school, with many religions and many nationalities covered - none have said anything to the child, parents or to school.
Whether the children continue to feel that way as they get older and into juniors, or whether they've change their mind, has varied. I don't know whats happened after primary as school is in a different location.
If the parents feel they need to withdraw their child because of it - let them get on with it. The school, and the remaining pupils, are no doubt better of without them in the school.