Basically, you would have independently set and assessed requirements. In the case of universities, it would be admission requirements, that the universities set and independently assess. Students, that want to get to these universities, then use any resource available to them to learn to those requirements. This could be through a mix of online and offline classes, remote learning, evening classes, day school, textbooks, tutors etc. "Official" high school is de-emphasized in this scheme, as there is no way it can provide the right thing for everyone. Nobody should be punished for school absences for pursuing their (academic) interests.
Schools should also do all they can to emphasize academics and de-emphasize sports, to counter-balance the natural tendencies of teenagers.