Oh I like the flashback... sheds some light on things which, depending how you choose to read it, clears a few things up, or makes them even more unclear
Is, for example, Mrs. Masters the instigator or simply 'following orders'... (not a request for clarification, just thinking out loud) - it's unclear, adding intrigue - which is good... and could easily go either way (although with comments like being 'aware of the risks' and going ahead regardless, I'm leaning towards instigator, for reasons as yet unknown) and curiosity about that and outcome will likely keep me reading...
That said, without wishing to detract from anyone else's enjoyment of the story, things like bad drug interactions & butt plugs would be a step too far for me... that's moving from 'merely' humiliating (for whatever reason) to, potentially, being very physically / psychologically harmful, potentially fatal even - not that I'm suggesting you'd necessarily go that far (unless you've changed your mind from when we were discussing ways Seth might be expected to react)...
e.g. A butt plug for a straight guy (who knows he can't trust anyone to keep it secret), is not something I could see any character willingly submitting themselves too, regardless of how keen they might be to graduate/take up a scholarship (a suspension of disbelief issue), whereas forcing/coercing the issue would, to me, feel too rapey... either of which would likely be the end point for me.
Although, even as I typed that the little hamster was running circles on the wheel in my head
- using the butt plug as a catalyst for something like
- She says something to the effect of 'you'll wear that, or else...'
- Seth says no, and storms out... in a blind panic, probably... worrying about what the 'sick bitch' is doing to him...
- Returns home to find school has pressured parents who, not knowing what's going on, get on Seth's case...
- Seth, not trusting parents, the school (remembering how the coach backed down), etc goes back with a plan, secretly records Mrs. M...
- Seth, says no again, storms out & goes to police and/or the press to **try** and get her stopped
- we find out a little more about the background / how highly this is sanctioned by the police response/newspaper article...
- and the story moves on from there...
She might respond to an article, for example, by revealing the 'legitimacy' of what she's doing, giving the story of all of Seth's 'exploits', spreading his humiliation far and wide (even if the story doesn't mention him by name, being a minor and all)... dropping Seth out of whatever 'this' is, opening the field for things to spread/continue whether he ever goes back to school / goes to college... and depending how 'malicious' you want to be towards the character, of course.
No reason he couldn't have a 'mental breakdown', for example, and still suffer at school/home - even if that does take some of the 'fun' out of it for the 'nurses'... oh, he's just doing whatever we tell him, and not reacting any more.... i think we broke our toy... or maybe he's just pretending...
or, (at the end of this story arc) a scene something like... Seth, all out of fucks to give, strips himself bare in the cafeteria... yelling out his angst... 'what am I doing? Who cares? You've all seen me like this already... enjoyed watching me suffering while the evil bitch ruined my life... that's it, laugh it up... but watch your backs... if she can do this to me, what's stopping her from doing it to you too... (yelling as he's dragged off)...
i.e. dramatically revealing what's been going on... and force her to 'read in' the rest of the staff/student body, some of whom take great pleasure in continuing/enhancing his torture (either continuing in the current story, or in a new, post HS environment)
Not that I'm trying to tell you how to write your story, do whatever you like to your characters - all I can ask is that you use your 'evil genius' to include a believable response (particularly to more extreme provocation) and then work your way around that as, fantasy or not, maintaining suspension of disbelief is important.
edited for clarity, hopefully.