Of course. I'm as old as dirt and we had kids from all grades in elective classes.
At our schools, English and the science and social studies recommended for freshmen are really the only classes that are single grade. In math, some come in having already had algebra, some double up and take two math classes while others take the minimum. Electives vary widely. My son is a senior who concentrates in fine arts. This semester he has Pottery I, so his class is mostly freshmen. He also has Art IV -- no freshmen or sophomores in there and there won't be any in AP Art spring semester either.