The problem is that it conflicts with what children are being taught at home and in school to help protect them from abuse, physical and sexual.
If this man wants to dress up in school uniform, yes, obviously he can. He can go out, walk around anywhere he likes, but if he has any common sense he must know that to dress up in the uniform of a local school and then walk close to those schools when the children are out and about, is likely to cause distress to parents and children and cause real concerns about the children's safety, even if it is misplaced.
Just because someone wants to dress as a school girl does not mean he can live his life as if nobody but hin and his preferences and desires matters.
The question I would ask is, how would the police and school respond were one of the maile teachers turn up for work dressed like this on a regular basis?