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
Catch up online with Wales This Week, Gleision - The Inside Story
The Cardiff Embankment scheme will stretch from the city centre down Dumballs Road to The Bay. The plans go before the council this week.