Lisa,
I have an older child DS7 who has very similar issues with crowded places and sensory overload. He also has the lack of friends. Have you thought about PDD? You have a lot of the SI issues, adding social cue problems. DS7 is gifted, and he does have imaginative play, so he is in no way a classic autistic, but he does have meltdowns when under stress and feeling overstimulated.
I would definitely look at the school and how they treat giftedness. Some schools that have wonderful reputations end up being bad fit because of their teaching philosophies. If they pride themselves on how disciplined they are, but you see no evidence of programs to help children achieve the discipline they expect, I would reconsider. Many gifted children have issues with school authoritarian mindsets and they do much worse.
Of course, if your child is PDD, a lot of structure may be exactly what she needs (quiet classrooms, rules that are consistent, etc). The biggest issues PDD children experience are with transitions. Things like changes in schedule (Mondays we usually do X, but this Monday we're doing Y), even if the changes are to allow them to do fun things, could cause meltdowns. Finally, just going to boarding school could stress her out because it is another big change, even if she chose it. She could eventually adapt, but the first year would be very difficult for everyone.
