I teach high school....in the suburbs, where education is fantastic. But I've also taught, worked with after school programs, and done research in urban schools. I'd have to say the biggest problem I have to say is FUNDING!!!! Or lack thereof. The US, as far as spending goes, is first in military funding and LAST out of all industrialized nations in terms of what it spends on education! That's just plain ridiculous if you asked me.
And as far as kids in urban/poor schools...how are you supposed to focus on math when you are sitting in class in the middle of winter wearing only a ratty t-shirt, haven't eaten breakfast...you know you have your free lunch coming up, but you have no clue if you'll get dinner that night? And you can't do your homework because you have to go home and babysit your 4 younger siblings and cousins while your mom is at work and your dad is out drinking (and then you have to deal with wondering if he's going to come home and beat you, one of your siblings, or your mom). Such was the life of many of my 8th graders when I worked in a city school last year.