“Class Divide” is a fascinating documentary that takes a look at the sharp socioeconomic contrasts in the quickly gentrifying Chelsea neighborhood in New York City between the wealthy students who attend the very expensive Avenues private school and their like-aged peers who live in the projects directly across the street.   Their stories and those of their families and neighbors interweave and the real story emerges — that these kids are not all that different from each other in their hopes, dreams and aspirations.