I've taught in a medium sized primary school with a PAN of 45.
They were arranged in 2 x reception classes of 22 & 23.
3 x KS 1 classes (mixed year 1 & 2). These classes were as similar as possible in terms of gender, SEN, ability etc.
3 x lower KS2 classes (mixed year 3 & 4)
3 x upper KS2 classes (mixed year 5 & 6)
The thinking behind it was that it provided greater equality to all children if the classes (apart from Reception) were all mixed, rather than have some single age classes, and some mixed.
It worked really well, with staff being able to work together on planning and assessment.
Latterly, the school has also introduced a system whereby after Reception, children stay with their teacher for 2 years to encourage consistency and deeper relationships, and try and lower the impact of the change of class each year with the resulting dip for a couple of weeks while the teacher gets to know the class. It's been received very favourably by the parents.