<p>MotherofTwo:
When my S took MVCalc at the Extension School, there were 10 other students from our hs who took it along with him. There were as well 12 other students from neighboring districts. As I mentioned, last year, there were 53 high schoolers out of a class of 83, mostly from neighboring districts.
Public schools, at least in our area, do allow for students to take advantage of proximity. But it is true that the classes are held in the evening (twice a week) and are on top of the regular classes. This is mostly a by-product of the "no study hall" policy enacted in 1993 by our state legislature. So, last year, my S ended up having 9 classes instead of the usual 7. The evening classes did not interfere with my S's ECs, but I could well imagine that for students having rehearsals until dinner time, there might be a conflict. My S did have some problems pursuing ECs because one class last spring and this past fall was scheduled in the late afternoon rather than in the evening. So far, he has not complained of too much homework.
As for acceleration in k-8, so much depends on serendipity. Some of my S's teachers were against "tracking" on egalitarian grounds, others were willing to accommodate him and were flexible enough to let him take a class at the high school. BUT, the elementary school was close to the high school, so he could walk there, together with another student taking 9th grade math. Last year, the school moved; it remains excellent, and my S's teachers are still there, but it is no longer possible to walk from it to the high school. I would not be surprised if the school sent the student to the high school by taxi if necessary!</p>