I get the whole let lesbians be lesbians thing but I'm not sure that's actually something that commonly tracks in ero games.
In this game the MC has already coerced repeatable groping sessions and blowjobs out of Debbie so it really wouldn't surprise me if there ends up being more activity between her and the MC. She may end up as bi but with a heavy preference for women if the headmaster and his psychological voodoo decide to aim for that.
I don't know how it would get there but I think I could imagine a scenario where Debbie would enjoy being dicked by the headmaster, for example if Sally is grinding her pussy in her face at the same time or something. Maybe the next stage before getting to that sort of situation would be to introduce her to dildo and/or strap on play and she comes around more to the idea of dicks being involved. idk.
I trust Altos though. If he decides to just let her be a lesbian, I'm sure he has some good stuff for us to enjoy without our MC getting too involved with Debbie. I just don't think we should write off the idea that the MC could eventually get his way with her because he can be pretty nefarious sometimes. He's not exactly respectful of boundaries.
It's not that common - but that might be one of the reasons people are somewhat tiring of it. The typical lesbian in games on here will
(a) be lesbian when it comes to love but will start craving dick
(b) especially your dick
(c) because it is just so huge when she got a peek at it
(d) so will her girlfriend
And while that is nice and dandy in terms of MC gotta fuck 'em all - and I am completely aboard - it makes for bad storytelling. Generally lesbians crave dick as much as straight males do - some might like to experiment with it a little but mostly it's a big no-no.
Of course the other option, pretty realistic in real life (rare still, but happens) is some kind of "whoring out" - doing sex for money or other advantages. This of course is an angle which could very well be applicable in this game.
I can't speak for Gato of course but for me the "dressing" - if stuff like that happens at all - would be important. Debbie shall never behave like Lucy. But I have confidence in Altos that he will pull it off, since the setting will allow for a "mutual benefits without any feelings" situation.
Personally, I also am a bit anxious about his (and Stagwood's) other issue with the too cute background characters. I want some background girls to not only sing in the shower but, ahem, sing a special rhythmic tune for me.