My son did PGL last year (year 6 mainstream school trip).
He coped well his 1-1 was there and they were very careful about who he shared a room with etc. He was very anxious beforehand but on the whole coped well, he hates school trips anyway so stress was always going to be high.
We had as a back up plan that I would collect him and return him in the morning if he couldn't handle the less structured evening activities but my services were never called on.
I sent him with key rings for each day with all the different activities on, the teacher said she walked in the room one morning and all the boys in his room were working out there day.
He did have a few incidents but nothing major. The only sad bit for him was that they made CD's for all the children with all the photos of there groups and rather than looking at it and remembering all the good things it just brought back all the times he had been stressed and really upset him