How I attended 2 high schools at once
The state of Michigan has what is essentially a gifted a talented program in the form of schools that specialize in math and science. Michigan's network of said schools is actually one of the two largest in the country, at least when I was still in high school, with about 13 different ones. (Virginia has the other, and they refer to them as governor's schools.)
Other states have similar schools, though most of them are boarding schools that specialize in math and science but still teach the other stuff too.
The reason I know this, btw, is because all these schools are members of a parent organization called the NCSSSMST (National Consortium of Specialized Secondary Schools of Math, Science, and Technology. Yes, I have that memorized.) The NCSSSMST held student conferences and I went to a few.
Anyway. The math/science schools in Michigan aren't boarding schools but rather half day programs that teach only math, science, and computer science. All other classes you take at a regular old high school. They were all county/area wide, I believe, or at least the Kalamazoo, Battle Creek, and Berrien/Cass county ones were.
So, I spent half my school day at the Math and Science Center taking calculus, physics, Comp Sci, what have you, and the other half of the day at my local high school taking my German, English, history, band, and whatever else. Separate set of students and teachers at each place, thank GOD, because I would've gone insane if I had just been at my regular high school. Thus, 2 separate high schools at one time.
To make things even more confusing, I was also taking classes at the community college. :)