Comment by Spivak

I think you're taking the wrong read on the situation, if you have a cross-section of your students that are failing a class that doesn't match with the roughly normal distribution you would expect then it means that it's a failing of the class. You have a bunch of affluent involved parents who are looking to advantage their child in any way possible teaching their kids algebra outside school hours. It's not a bad thing, it's actually great that parents are involved, but it means the class isn't doing anything one way or the other so why even have it?

You only see this kind of behavior in math where there's an on-paper advantage to be conferred by letting kids jump ahead, you don't see it with social studies or english.