While the birth rate, on average, is falling and affecting some things, such as the school you mention, I don't think it's widespread.
I live and work in a town in the North West. It is very culturally diverse and this has become more so, even since I was a child (I'm 36 now). We have a large white British and a large Asian population in the town. In recent years, we have also seen an increase in the number of African families settling in the area.
One child I teach for instance, migrated from Nigeria only 2 years ago.
Obviously among all of these people there are varying numbers of children - anything from 1 to 7/8/9.
So, for towns like ours, the population isn't declining, whereas in other areas it may be.