Not limiting myself to just high schools:

Elephant in the room in my state is definitely chronic absence. Depending on source it's when student misses something like 15+ or 20+ school days a school year. More affluent areas have numbers 15% and lower. Less affluent ones it can be well above 50%. And nobody is doing anything.

Test scores substantially mirror this bifurcation.

It is substantially worsened by charter and voucher schools. Which interact with the whole mess in complex and negative ways.