Mine went to ASP. The younger (who was 12) slotted back into the UK system with no difficulty at all, and is way ahead of her peers in most subjects (particularly French), according to her teachers, and predicted a string of A* at GCSE. This surprised me as most people gave me gloom and doom over the American system and told me she would be at least one year, if not two, behind. She is in the top group for everything and wasn't in the top group for anything at ASP!
DD1 was a little trickier, as she was older (15). We thought/were told it would be difficult, if not impossible, for her to go back into the UK system, so have kept her in the international system (at VAST expense!). She is more than holding her own academically, and is doing her IB French a year early - her French teacher said that she is virtually bilingual and that her written French is better than the Francophone students. (We were, on the whole, very pleased with the standard of French teaching at ASP, although some of the French teachers were not particularly pleasant...)
I think if they are in a different system and transferring to the UK, there comes a cut off point where they will struggle, which in my opinion, is about 14/15 - when some kids will already have taken some GCSEs and all will be embarking on their GCSE courses.