15 in DS's private school class at present; the school imposes a maximum of 16 in all classes. State schools here (Scotland) have a maximum of 25 in P1 (K to you) classes, usually in the high 20s thereafter I think.
TBH I find it hard to be excited about the differences in the 25-30 range, which seems to be where the political interest is here. Differentiation is, I think, more of a reality here than it seems to be in the US (teachers are assessed on it routinely) but it usually just means there are high, average and low challenge versions of each lesson. Being more individual than that is already pretty impractical once you have 25 in a class, it seems to me. My feeling is that 15 is much better.
