The best school system I ever attended rigorously divided classes between social and academic --- for social classes (gym, health, social studies, homeroom) one attended at one's grade level, while for academic classes, (English, other languages, science, math) one worked at one's grade level (but with a cap of 4 grade levels through 8th grade, so a 4th grader couldn't take high school classes) --- after 8th grade this cap was removed, and students could take any classes which they could qualify for academically. To facilitate this, some teachers were also accredited as faculty at a nearby college, and if necessary arrangements were made either for college professors to come to the school and teach, or students travelled to the college.
Most students graduated with at least a little college credit, and many received a college degree along with their high school diploma --- until the Mississippi State Supreme Court decided that it was illegal because it conferred an unseemly advantage on some students with no corresponding compensation for students who were unable to avail themselves of it academically.