I wouldn't suggest it for grade school, but at least for high school, why not? No grade levels, you have 3 quarters (and use quarters, semesters are just a cost saving measure) per regular year, 4 years.
(FWIW, a quarter of math in a community college is roughly a year in high school.)
Tell students that they have 3 quarters of math to complete at minimum in 4 years. If they fail one quarter, there is lots of time to take it again. Why have the idea of grade levels at all?
Remove 80% of responsibility from teachers for passing or failing, remove all standardized tests, and let's go to a model that we know works.
Edit: To clarify, I am not suggesting we import the ivy league "Make half the students drop out through stress" system. I am proposing a quarterly system of learning where students are free to take the level of course work that is most appropriate to their needs.