Hi,
I can advise you Horizon Japan International school which has branches in both Tokyo and Yokohama. I have some friends whose children are attending to that international school and I can easily say that all of them are highly pleasant about the school's education. Although this school is new as far as I know they showed a great performance to prove themselves in a very short time as my friends mention. For your reference below is the school's website:
Horizon Japan International School - Tokyo and Yokohama
Caner,