Teach them more skills and/or use the extra time they do not need on their strong sides to boost weak ones with extracurricular activities.

Yes, you cannot skip a grade, but nobody is stopping a kid from going to a later grade for some classes really. The school social atmosphere has to be right for it though.

But nobody wants to pay for it.

> nobody is stopping a kid from going to a later grade for some classes really

Nobody should be, but many people are.

At a minimum, the college-style model of subject-based classes and prerequisites for those classes should start much, much earlier, in elementary school.

There are elementary-school students who should be in calculus classes, and there are high-school and university students who should be in remedial arithmetic classes. (Though in some cases the latter would be less true if K-12 hadn't failed them so badly thus far.)