A school can worsen manners (I'm not sure what "social skills" means) if it tolerates misbehavior by students.
I think there's the issue of "social manners" vs. "Street smarts". Kids need to know both. I learned manners at home. I went to a ROUGH high school. I mostly stayed away from trouble, but I quickly learned when to speak up, shut up, and get away as fast as possible. I do think these experiences were invaluable, but I shudder to think of my son learning them in quite the same way.
The social landscape of childhood and especially adolescence is ragged. I think offering our kids experiences where they can learn how to survive is necessary whether in school or out.
