What is it you fear? I'm presuming this is around children's safety so what do you think he's going to do?
Fear? Let's start with the mental health, fear, anxiety, discomfort of the young kids involved. He is a grown man wearing a school girls uniform deliberately placing himself in close proximity to children.
What do I think he is going to do? Precisely what he IS doing. Making countless kids, and parents, uncomfortable. I wouldn't assume his behaviour will escalate, but it probably will, once he become acclimatised to doing what he is currently doing (like the report that he was trying to climb the school fence again yesterday)
What do you want/expect the Police to do? Arrest him, detain him? What offence?
If he has mental health issues and capacity then any kind of order barring him from being within X distance of a school. As happens with many other people for very many offences or potential offences.
If he has no capacity then a better community care option. But we know how likely that is. Especially when so many place his freedom of expression above the safety and comfort of others.
Would it make a difference if he was younger or didn't wear the uniform (but other teen clothes)?
No. Because a man with no kids loitering outside school gates is usually taken to be there with ill intent. This man is different because.... he wears a girl's school uniform and that somehow makes it all absolutely fine and dandy.
Double standards for trans identifying men is not acceptable - no mater what other mental health issues it is speculated they may have.