There's a whole pantheon, just to list a few: teaching to the tests, underpaid/appreciated teachers, an emphasis on educating for employability instead of critical thinking and on top of all that a lack of coherent structure to better mete out resources and standards.

Reforming public education in this country is going to be an absolute beast, though I often think public schools are better than a lot of the criticism flung their way would suggest, there's still a need to drag education into the 21st century