For the corruptible male character, we nominate Alex Wolff (pictured here on the left), Nat's younger brother.
For the corruptible female character, we nominate Kiersey Clemons (we know you're better than that, Kiersey, but this is a fantasy cast, so we're aiming high).
Bash Casey (aka Bash Hargrove-Valmont) didn't grow up in the privileged world of his parents. Instead, he was raised in Kansas, where kids do oxycontin in cornfields rather than snort cocaine out of crosses (not to stereotype or anything). We imagine he's good looking, but naïve and a bit of a muppet when it comes to social situations. In our version, he's played by Jude Law's mini-me Rafferty Law.
Not to confuse classics from America's cultural canon, but we imagine Bash's love interest as something of a Daisy Buchanon, not entirely a terrible person, more of a vessel through which terrible things happen. She's got to be beautiful and a little bit out of reach. We'd like to suggest Odessa Young.
As the Annette/Madame de Tourvel of this story, Bash needs a mentor: someone cunning, handsome, rich, and a bit of a sociopath. Scream Queens' Lucien Laviscount, perhaps.
And for the teacher, because there is always at least one teacher in boarding school teen dramas, please, NBC, let it be Joshua Jackson.