My daughter graduated from Capistrano Valley high school in 1990. When she took algebra in 8th grade, 40% was a passing grade. When I asked the teacher how she could consider that the kids had mastered the subject with a 40% and move on to the next lesson, she said that all the bright students take algebra in 8th grade and that she had to stick to the curriculum and could not take the time to go back over anything.
So, I tried to help my daughter at home, but the text books did not contain any examples, just practice pages. I assume that the lack of examples was purposeful, so that kids with interested parents would not have an advantage over those who did not. It was quite common for parents to hire an algebra tutor to teach their kids outside of school because they didn’t learn anything in school. Of course all the tests were multiple choice scantron tests, anyway.
I went out and bought another algebra curriculum so that I could teach her at home. It was really tough because my daughter was an elite athlete who missed a lot of school for competition outside the country.
At the beginning of the year, teachers are handed a district "pacing guide" (aka racing guide) that tells then what page they will be on on every day. There is no time for remedial work, no time for extra explanation or examples, no time for anything. This is not teaching.
As an aside: In order for Special Ed to have obtained mastery, they must have 85%. What kind of system requires special ed to have higher standards than gifted students do?