I have friends whose children have joined year 2 in the UK after being in the primary years programme, IB, play based learning etc abroad.
The skill sets these children have got are good and they have been taught how to think. The children I know of I would say are above average academically.
They've found they haven't covered the same ground academically, in terms of the formal 3 rs, so have been placed in the lower ability groupings/tables/sets.
Not a problem? They'll find they're own natural level academically in time?
Interestingly formal learning doesn't start until 7 where they've come from and I'm hugely impressed by the children. They are happy, confident individuals.