Unions NAHT Cymru & ASCL Cymru have written a joint letter to WJEC, asking it to waive fees for re-sitting GCSE English Language units this summer.
"We have to report that our members are extremely dissatisfied with the position that their staff, pupils and parents now find themselves in" said the letter.
"This is a highly unacceptable state of affairs as young people begin to prepare in earnest for the challenges of the array of summer examinations."
The National Union of Teachers says it has also asked WJEC to waive fees to re-sits this summer.
More top news
The Bank Holiday is here! Which is usually a recipe for wet weather! Yes, we will get our fair share. But it's not all doom and gloom!
Carl Edwards goes back to his Mid Wales roots to show what his hometown has to offer
Wales flanker Dan Lydiate believes tomorrow's Dublin appointment with the reigning RBS 6 Nations champions is "brilliant preparation".