I'm a lower on near full FA at Andover, so I can answer this question.
There definitely is no major divide between poor kids and rich kids. In general, it's very hard to tell who's on financial aid, which is a very good thing. Also, since most activities you participate in don't cost money, financial aid kids are rarely left out of social events. If you go to Andover and are on financial aid, you won't have any major social problems.
With that being said, the environment here is slightly preppy. People dress much nicer here than they do at my old public school. You don't see as many grimy jeans and sweatshirt people around here.