Which is which is stated on each, because only one of them is safe to quote publicly today.

Primary schools, nationally

CSO open data. Average class size runs from 2003/04; classes and pupils from 1999/2000.

Loading education.json

Series values by academic year
YearValueRelative
An average is not an experience. Average class size is a national mean across mainstream classes. No particular child sits in a class of 22.2, it tells you nothing about any school or county, and it excludes special classes and special schools. A mean that moves by a fraction of a pupil can still hide large changes at either end of the distribution.

Post-primary schools

Sources

What this does not show