Hi Agent99!
My son is now a senior in college, but we had similar challenges. When he was in sixth grade, he started taking math classes at our local community college. There was no way he was passing for a college students. He was around 12 and pretty short. I was very concerned that during down time (like waiting in the hallway for class to start) he would be surrounded by sex-talking older teens, but this wasn't the case. Maybe it's because he took mostly math and science classes, but while some students were curious about the kid in class, they were friendly and helpful. They didn't care if he was assigned as their lab partner. I think they got a kick out of him. Eventually, people got used to him being there.
When he was in seventh grade, he wrote a letter to the presidents of the two four-year colleges near us telling them about his plight, his huge thirst for math and asking them if he could simply sit in on one class each semester. Both men agreed to let my son take one class for credit each semester, which thrilled him. So for the next few years (until high school when he split his days between high school and college) he was able to take the calculus series and some physics. It was great for him. He got his first "B" in physics II and discovered that he didn't know how to study. He was frustrated and challenged and thrilled.
This ended up being the way we went. He did also skip grades, and it was sometimes hard socially -- in some ways. But it had been terribly hard socially when he was unchallenged and miserable with his same age peers. Sometimes we couldn't address both issues, so we chose what was most important to him, and that was academics.
Maybe you could try to community college for classes? We didn't need any special permission. We just walked in and signed him up. At first, he took evening classes. There was never any issue with him hanging out with inappropriate kids. There was a mix of college kids and adults. Quite frankly, who would "hang out" with a twelve year old?
At community colleges, they usually have math starting right from algebra, so you child could jump right in. The nice thing is that they move more than twice as fast as the high school classes!
