We had a similar dilemma fairly recently.
Ds was at a lovely small school with small classes, which was one of the reasons we liked it.
However, a few people left, making the class even smaller, and that's when problems began. Although he did have a best friend, the friend, quite understandably, didn't want to play with ds ALL the time. So ds became very unhappy.
We've now moved him to a slightly bigger school. Class size is still not enormous, but the benefits have been huge.
With a few more boys (girls don't really feature now ds is 9!) there's always someone to knock about with. If he falls out with one friend, or doesn't want to play a particular game, then he has other choices.
It did take him (and us - we'd been at the other school for ages) a while to settle, but now he is so much happier.
If you have alternatives, I would definitely think about moving dd - particularly if you have other issues with the school also.
I hope all works out