For sure, there are a lot of all girls schools that have male teachers. Men have a different thought of thinking and having male teachers can give other views into something then only having women.
The gender of the teacher isn't a problem at all, as long as any teacher behaves professionally it won't lead to any problems, just like in any other school or situation in general. I guess being a female teacher in an all boys high school is another thing with all the horny teenagers. Also when you completely separate the genders in only leads to further problems and more misunderstandings then there are already.
I may have misunderstood your tone a bit but it kinda sounded like you were saying that these adult men would behave like animals and jump onto the girls in the school. Men are not something evil that needs to be locked away, they are just doing their job in the schools, there is no need to see something bad in any situation.
Yes, as long as they act professionally, that's what we all want in teachers, yet some school boards today either have no idea how to recruit those kinds of teachers or they're only recruiting those who can fit into the agenda of what they're trying to 'teach' in their schools. For an all girls/boys school, one would wonder how or why the school board would enlist teachers that don't fit in the community that was blatantly clear of what demographic they're teaching. Sure, having different opinions or perspectives in things can be a valuable asset to have, but when you're in a community where you clearly stand out in some way, especially one in Japan that's more collective as a society than independent, there's going to be at least
some discrimination that follows them.
I wasn't even assuming these adult men would jump onto those high school girls, either, they just want to do their jobs and do them well. Despite that, there are girls who could give these male teachers problems, for one reason or another, and if the girls don't get their way, she could very easily call him out on a crime he didn't commit and ruin his life and teaching career for good.
To be clear, I don't see anything wrong with a teacher being in a school that teaches entirely of the opposite sex. I'm not trying to see this or play this out as a bad thing. I'm just saying that the teacher in question should know despite what they're trying to do in their position, they should take note of how they differ from the students in question, and approach their workplace with caution, because they'll never know what kind of person will be out to get them for those differences alone, and be prepared to defend themselves with professionalism when that happens.
If these girls can get along with this male teacher, great, that's a welcome surprise in my book. There should be more of that when the complete opposite is usually shown in today's manga. I understand that's usually the case to provide some form of entertainment to the readers, but that doesn't mean it should always be that way.