tbh. if this was all just a matter of "Leah or Susan" I wouldn't mind dropping Leah like a hot potato, but it's fairly obvious that pretty much everyone will hate your guts for taking Susan up on her offer. I could imagine Melody going "dick move, apologize to Leah or something" and then not caring that much anymore and I assume Hailey might come around anyway because she is less involved, having heard everything about the teacher second hand, but Leah is definitely out, Karina actually cares about her sister so she is out and there will likely be issues with Alice as well, since she didn't get along with Susan either.
In a direct comparison between Leah and Susan though, Leah has no chance whatsoever. Obviously in real life what the teacher did would be fucked up and you'd be insane to try and continue contact with her, but this is a story. And as far as the story goes I prefer characters that I can either like or at least love to hate. Pretty much all other characters are fairly easygoing and likeable, Susan started out as weird but then jumped into the "love to hate her" category when she randomly showed up and ruined the MCs day. But Leah? Holy shit, I cringe whenever I see her in a scene because she has been a nuisance since day one. First she's pissed at you for supposedly being Alices boyfriend, then she's pissed because of the Teacher, then she's pissed because of her family, then she's pissed at her sister, then she's pissed at Hailey, then she's pissed that the other characters like her sister. I absolutely don't care about Karina because she looks disturbingly young to me but at least she actually adapts to the group a little and tries not to be a downer, but whenever Leah opens her mouth I sit there going "let me guess: you're angry/tense/annoyed because of something again and will try to ruin everyone elses mood?".
It made sense to me in school, since she was afraid someone would "snatch her gf" (which kinda did happen) and she obviously had a reason to be angry at Susan, but I had hoped she'd actually calm down a little after that instead of still being the main source of pretty tedious and boring conflict almost every other scene.