This is basically what my experience was like as a home-schooled student. A lot of self-guided learning, school days significantly less than 7 or 8 hours, and lots of extra-curricular activities.
I went to her school and asked her about special programs for her, potentially skipping grades, etc. Their response was essentially that in a couple years time the other kids would catch up. Well, of course the other kids will catch up, you're going to tie a big anchor to my daughter that she has to drag through a bunch of boring curriculum she already knows.
One could argue that high school is a bit unnecessary, and a good move would be to simply skip it.
I do have a friend from Israel who got his Bachelor's degree at 18 at the same time he finished high school. I can't see that happening in the U.S. system.
The first two years of undergrad at UC, by design, are for the general education requirements. You're not even supposed to take classes in your major until year 3. It shouldn't be hard to skip much of that.
I didn't know at the time that they kick you out after a certain number of credits (around 240?), and this actually affected my choice of double major. I took a watered-down "linguistics and computer science" BA as my double instead of the full "linguistics" BA, partly because the unit cap was bearing down on me.
Edit: it was "watered-down" because it required a ton of CS classes that I had already taken.
Further, finishing most of the physical sciences requirements in high school meant that I wasn't exposed to some awesome professors in college. I imagine that some students who might have a deep interest in chemistry but a disinterested high school teacher never bother to explore the subject in college if they pass the Chem AP test.