This really isn't normal behaviour, and I do think the unisex toilets is a bit of a red herring. I work in a school which has one set of unisex toilets, the rest single sexed, and it honestly works fine. I think the key is giving students options so they can chose what they feel comfortable with.
The issue is the boy feeling entitled to take photos of the girl. If he wasn't doing it in the toilets, he would be upskirting on the bus, or cornering girls in a less visible part of the playground, or something else.
Let's be realistic, you don't go from being someone who's interacting with girls normally to kicking down a toilet door just because the toilets are now unisex- there's more to it than that.
Sexual harassment and assault is becoming a lot more common in schools, and removing unisex toilets definitely won't solve the problems. Schools really need to take a zero tolerance approach. Far to many approaches seem to focus on rescuing the boys, rather than keeping the girls safe. This won't be a first offence, and he probably should have been excluded from the school by now.
The police also tend to be very unwilling to get involved with sexual assault that's happened inside a school- even if it's fairly obvious a crime has been committed. Even when there's good CCTV evidence, or another student has filmed what is going on, they're often just not interested and want the school to deal with it. Or they'll "have a word" with the boy, but actually take no action.
Personally, I think we need to be dealing with the growing issue of male violence in schools, not focusing on toilets.