How can I negotiate with my math teacher?

Colleges ask for unweighted GPAs. It's up to your school whether or not they think you're ready to take a cl, and they decided you weren't. It's far too late now since the fall semester is about to start. If you're good at math, great. Do well in this cl and take something else next summer. But being a year or two advanced in math is very common among college-bound students, so you need to do a lot more if you want to stand out to a top school.