The National Union of Teachers says it is concerned that in six local education authorities - Conwy, Caerphilly, Merthyr Tydfil, Wrexham, Ceredigion and the Vale of Glamorgan - the number of junior school classes with more than 30 pupils on the register has already increased.
NUT Cymru secretary Gethin Lewis said: 'According to figures from the National Assembly, at the moment there are approximately 1,700 primary school classes in Wales where there are more than 30 pupils.
'We are calling for an automatic trigger so that every time the number of children in the class exceeds 30 a new teacher is employed.
A spokeswoman for the National Assembly said that when they had committed to the pledge to cut class sizes at reception age they had stated it would not be done at the expense of ket stage two pupils.
'There has been a reduction in the number of classes of more than 30 pupils at key stage two in the last year. The number has fallen from 38% to 31% across Wales.
'To employ extra teachers when a class goes over 30 is not feasible because the increase may be a temporary measure and what would happen to the teacher if the number were then to fall below 30?'