There is currently no limit on the size of most primary and secondary school classes in England although infant classes – for pupils aged four to seven – have a statutory maximum of 30 pupils.
Some exceptions are permitted, such as including a child with special educational needs who is partly educated in a special school as the 31st pupil.
Increasing class sizes "impose greater workload on teachers, which is unreasonable and unacceptable and must be resisted", the NASUWT union say.
According to official Department for Education data, 39 primary schools in Calderdale have classes with more than 30 pupils.
In total the schools are home to 65 classes with more than 30 pupils, containing 2,132children.
These are the Calderdale schools with the highest number of classes which contain more than 30 pupils according to the data.