hi! KJS is a nice school! my son is presently in Y1 and he's doing great! he loves going to school, and he's learnt to read and write since he's been there!
if your son is in a UK preprep school, i dont' think he will have any problems settling in at KJS. they are in the midst of going IB (PYP) and i'm very happy with my son's progress so far.
one thing though - the school will be rebuilt soon and they may be moving to a temporary site for the next 2-3 years - we're still waiting to see where the temporary site will be! but apart from that, the teachers that i've come in contact with are very nice and caring and i've noticed the children and are all enjoying their new school life and learning well.
i think it's worth a visit during a school day to see the atmosphere and feel it for yourself! if you have more specific questions, i'd be glad to answer them to the best of my knowledge!