I'm ok with that kind of roleplay; I enjoy submissive roles like that sometimes so I don't find it strange that other people like the dominant bits, and I don't think someone who wants to act out a schoolgirl/teacher fantasy with a grown woman is in danger of wanting to move on to real schoolgirls.
There are other things, like "pretend to be my step-daughter", that I'd find more worrying. With any fantasy where the reality would be harmful to another person, even if it's an unusually creepy one, I think it's a pretty small minority of people who'd consider doing it for real and playing it out with a consenting adult is completely different. However, I can see how if someone was capable of doing it for real their desire may be reinforced by roleplay, so there are some things I wouldn't want to act out with clients just in case.
I did do one schoolgirl/headteacher roleplay that still makes me feel icky when I think about it. He was playing it in a nice grown-up way, like someone who wants to keep a young child on their side. I don't think he actually had paedophilic tendencies, but it was way creepier than the punishment-themed stuff. If you decide to go ahead you might want to ask for a brief synopsis to check that he's not likely to put in something that'll squick you out.