Yes it has happened in our school.Ours is a large Primary and we sometimes have composite classes.We have needed to do this for 5 and 6 one year...........3 classes of Yr 5and 6.

There are legal limits to class sizes in ks1 which as someone has already stated is 30. No limit in ks2.
We have 39 in year 2 this year so we needed an extra class and teacher. One is a straight ye 2 class of 25 the other a mixed Y1/Y2 class of 26.

Also someone has said it is based on floor space and that is also correct. One year we ended up with 32 in a ks1 class as we lost an appeal due to floor space so the head was expected to arrange the classes [IE create a new class but with only two children over the limit the budget would not stretch to another teacher. So even though the legal limit is 30, it doesn't always work out like that.

Is that in every LEA bluebell

as a governor in my previous school I remember setting the limit every year, KS1 was 30 and KS2 was 35, cant remember why we set them but seem to remember it was something to do with funding

On the odd occasion we wanted to go over that number it would only have been possible if we had a child who was working in a Y1 class and he was a Y2 child, he would be registered in Y2 but worked in Y1 then we could get away with an extra child in Y1