Ideally, the public school system should be able to provide an education for 12 years at whatever speed a student can handle the courses, even if it means graduating with a bachelor degree or trade diploma.
Back when our kiddo was in 2nd grade, most in the class were far advanced in reading. The teacher let those students go to the library and read at whatever level they wished while she stayed behind in the classroom to teach reading to the other students. Many had their noses buried in Harry Potter. So, great idea, huh! Nope, the teacher was fired for not holding the kids back with the lowest common denominator group.
FYI, the advanced group all attended a little part time church pre-school together. All but one were honor students graduating from high school bam, bam, bam in gpa.