The neat thing about stratifying by ability is that it doesn't require any additional resources or special programs. If you have 20 teachers in a school, you divide the student population into roughly 5%-wide ability bands instead of roughly 5%-wide age cohorts with random ability, and that's it.

Mastery learning is also more effective than moving on with knowledge gaps, so this should be expected to raise everyone's outcomes.

> I want every student to have a good education in math, even though it is hard to do that.

That's a laudable goal but I think it backfires in practice: a lot of students struggle with math and consider it to be torture, and will rarely require the skills and insights that learning algebra, trigonometry, geometry, and calculus will offer. Having done that work I find that I use very little of it in my day to day life (personally and professionally (as a programmer)).

I'm not suggesting that path be eliminated, only that it be an expected track for those interested in a STEM career.

For those who are not, just teaching them math literacy that can be used in contemporary daily life (some statistics, math reasoning (investments and debts), etc.

I love math -- it's the language of the universe! But it shouldn't be used to torture kids who will only learn to say "I hate math".