I'm not sure how age-based progression ever became the standard, but it stinks.
Offer students a comprehensive mastery syllabus that they can navigate at their own pace (possible within a normal school setting without anything too radical) and let them progress as they master skills. If you're 15 and still working with a bunch of 11 year-olds on Arithmetic 3, so be it.
Caveat: keep the overall age limit on schools (ie you have to leave high school by 18 or whatever) but students can carry on their work elsewhere, if they wish.
[Again, not really in scope or on topic, but I beg your indulgence.]