No, they can't. Tell them to fuck off and threaten with reporting them to the BoS. State that they can't do what they're planning. They know perfectly well what is within regulations and what isn't.
Play hardball with them. Stick up to them. It's a disgraceful how higher ranked schools pull shit like this. In the ubercompetive world of today, they're forgetting what the point of education is: to educate.
This kind of protest only works to a point, from there on it's just pointless hand-wringing. Look, schools are perfectly within their bounds to bar someone from a particular subject. Let me tell you that this isn't only commonplace at higher-tiered schools; my school was just an ordinary, salt of the earth Catholic school and these kind of standards were
religiously (no pun intended) enforced. There is a reason for this. Yes, schools should be about education, but at what point does education become daycare? At what point does one's education become diminished because the teacher is spending too much time with one or two particular students? No disrespect or reference to the OP, but these standards exist to stop people who'd otherwise struggle in courses that are notorious for difficulty. So in short, yes, schools can and will block people from courses they deem too difficult for certain students.
Now, don't get me wrong OP. I'm not saying that you're going to struggle. On the contrary, I think if you've been getting A's and B's
consistently, then you'd probably excel in said course. Hell, one of my closest friends had to really hammer away at our school's Maths Department to get accepted into ME1, not to mention needing to complete an additional yearly exam. In the end, she turned out to be one of our highest Maths achievers. If she can do it with
a lot of hard work, then I'm sure you can too!
I strongly disagree with Kolmias, the nuclear option is very unlikely to work in my honest opinion; all you're going to do is piss the school off with a protracted argument testing the mettle and patience of both yourself and the institution. I think it would be better to try an assuage some of their concerns. Draft a formal letter outlining the reasons you'd like to do the course, stating your intention to lift your game and try very hard. Say that you'd be willing to be placed on 'probation' so to speak, prepared to accept the need to achieve a certain mark or standard by your half-yearlies and that if you fail to meet the goal, you'd reevaluate then.
Bribe them, bargain with them, barter with them. But most certainly
DO NOT anger them, you're only ruining your own chances if you do.