Schools segregate on age now not ability.
For smart kids, school is frustrating and holds them back. It is better for them to leave school at 15 and transfer to a community college where they can take control of their education.
For not-smart kids, vocational education from 15-17 leading to apprenticeships gives them something of interest and useful to them.
For average kids, the system might work if the other two groups who are frustrated were channelled towards more useful pursuits than sitting bored, and sometimes forced to take amphetamines like Ritalin just to have the appearance of being focused.