Exactly, many of these students are doing very poorly in their other subjects (not all obviously, but we don't destroy the educational system over outliers) but

Exactly, many of these students are doing very poorly in their other subjects (not all obviously, but we don't destroy the educational system over outliers) but are just being passed along. High school algebra is pretty basic, OK. I think we all know that if we are honest. In some ways, it's just too simplistic, many other nations would agree wholeheartedly. Students that are failing these classes, I think, for the most part, have other things going on, perhaps other educational deficiencies, problems at home (even if it is just a bad study atmosphere), unable to prioritize, caving into other pressures of just giving up, etc. Parents have to be engaged in education. It's much harder without that. I suspect without parental engagement, those that succeed are really motivated self-starters and autodidacts, outliers.