This sounds weirdly punitive. Even if there were an ongoing behavior issue (which it doesn’t sound like you perceive there to be), restricting a student’s progress in an area of strength, such that he is under-challenged and unable to feel a sense of accomplishment, is only risking making the behavior issue worse IMHO. I wonder if they’re hoping you’ll switch schools if they’re uncooperative enough.
I had a similar issue with my daughter in 8th grade, although in that case it wasn’t specific to her; they just decided they didn’t want to offer Geometry that year, and instead put her in a bizarre advanced math class taught by a couple of volunteer parents who just did whatever they felt like. She decided she’d rather do BYU geometry in order to move on to AlgII in 9th grade, but she never finished the second half of it, and it dragged on throughout high school and disrupted the college application process (because she couldn’t attend a UC without it). It was all really unnecessarily annoying! In reality, it would’ve been okay for her to just take geometry in 9th, but my older kid had had such a negative experience in that class at the same school (due to no honors track until AlgII) that she didn’t want to. It was the gift-that-keeps-on-giving of gratuitous aggravation.
Given that you’re talking about AlgI in 8th, Geometry in 9th… that really does make a difference, as he won’t get to Calculus in HS unless he doubles up or does a summer class later. I wouldn’t be happy about acquiescing to what they’re trying to do, given how advanced he is in his outside classes.
When you say that BYU won’t work, does that mean that no outside class will work, or are there some that will? For example, could he take a summer class at a private high school in your area (this is a popular option where I live, especially for Geometry which isn’t hard to finish in a summer), or an online class from a vendor that would work?