I don't think that occurs because of a lack of social skills in kids attending Deaf schools, but more just a personal comfort level and preference. Studies have shown that kids attending Deaf schools have greater psycho-social adjustment and better social skills than those who are isolated in the mainstream.
Deaf/HH children can develop more social skills in general at a Deaf school. I was just talking about in particular being used to and knowing how to deal with the situation where everyone is hearing except you. I totally agree that a Deaf environment helps foster socialization because the communication barrier is lifted.