While I agree in principle that this is likely (if with "everybody" we restrict ourselves to the major characters with their own info sheet), this isn't predominantly a "harem/trainer" in the classic sense with having sex as the main goal (though our headmaster slips up about wanting this as well in his inner monologue), so I wouldn't be surprised if a few dodged the bullet - I actually kinda hope so for our prefect Debbie because I am not too sure about how I would feel about too much extortion to coerce her while I know how I feel about "curing lesbians" - don't like it. However, if written well, also that would be a possibility.
Yeah, I see Debbie as about a 5-5.5 out of 6 on the
Kinsey scale. She is pretty hard lesbian, but she is not 100%, and has a bit of penis envy. I don't think she will ever be "cured", as it were (at least I hope not, that would not be good writing), but she will likely have sex with him as part of a group event or something, as long as she gets to play with the other girls too. She won't make many exceptions, but I assume there will be one for the HM at some point.
I don't like him pressuring her though, so I hope it is more her idea.