So say there’s a P2 class of 30, and then 35 P1s. An additional 5 kids isn’t enough to fund a whole extra teacher and there isn’t space spare to put them with the P2s as that’s already full.
Sorry I’m just not getting how in the Scottish system anyone can be guaranteed a place in their catchment school.
In that case you'd have a 25 P1 class, then a composite P1/P2 class. After that it would depend on numbers in other year groups. The last school I worked in had P1, P1/2, P2/3, P3, P4, P5/6, P6/7 and P7.
Obviously you get years where space is s problem and you end up with portacabins, but often you get years where it flows. So two P7 classes go up to high school, but there's only one P7 the next year.
You are guaranteed your space even if it means building work or portacabin classrooms or whatever.
I don't think it would work in England because I think it would be too big a change, same as the English system wouldn't work here for the same reason. Most of the time it just works.
Looking at where we are and where MIL lives I think there are more new schools as houses are built. That's just two towns though so I don't know if that's commonplace. We've had a lot of building in the last 10-15 years and there's four new primaries (one a replacement that doubled size) on three campuses (one is a non-dom and catholic school on one site) and one massive new high school.