Head teachers are warning they may have to lose staff to pay for a new workload agreement for teachers.
Duan Evans said he may lose a member of staff
From September, every teacher has to be given half a day a week away from the classroom for preparation.
Duan Evans, who runs Ysgol Gymraeg Abercynon, said he was considering having to let a member of staff go to fund cover for these half-days.
Welsh Assembly Education Minister Jane Davidson said there was already enough money for the deal to work.
This week, the National Association of Head Teachers (NAHT) pulled out of the agreement.
It said the Welsh Assembly Government would need to put up an extra £77m next year for it to work in Wales - at the moment, £58m is being given.
It has warned over possible cutbacks, but Mr Evans went further, saying the funding was "simply not enough".
"There will be cuts in all areas of school life and standards will suffer," he said.
"It's an absolute nightmare for any school, whatever the size, whatever the situation and the structure within the school.
"The main gist of this new PPA [planning, preparation and assessment] time is to raise standards. I find that very difficult to swallow to be honest because I don't know how we can raise standards if we have to cut in other areas."
Ms Davidson said there was enough money and it was up to head teachers "to determine how to deliver this statutory responsibility".
She said other schools in Wales were using the new agreement to bring in a range of creative and sporting activities.
"[They] are providing a really exciting enriched curriculum for all pupils in the school...on one afternoon a week, which fulfils everybody's planning preparation and assessment time," she said.