Staffing shortages on the play-based foundation stage for three to seven-year-olds urgently need to be overcome before the scheme is fully implemented in 2008, headteachers have warned.
The situation is so bad in some areas that pilot schools have been allowed to employ unqualified staff "as long as there is a commitment to training".
Angela Jardine, of the General Teaching Council for Wales, says some schools are struggling to bring the ratio of adults to children to 1:8, the existing ratio in pre-school playgroups. The problems are particularly acute in the Welsh-medium sector.
"Along with the requirements of the workload agreement, this problem is going to provide a real challenge to implementation of the foundation phase," she has told the council.
Meinir Morgan, head of Ysgol Gynradd Ffairfach, near Llandeilo, is predicting a continuing shortage of suitable support staff in the run-up to the national roll-out of the foundation stage, expected in 2008.
"A lot of young parents are working these days and when schools, particularly in rural areas, need an extra care assistant there is no guarantee that such a person will be floating around the school gates.
"When our numbers crept up to 31 last June we had extra money and could afford a care assistant for some weeks. But that cannot be the case in all rural schools and I foresee big problems."
Her views were echoed by Ann Davies, head of Ysgol-y-Dderi, in Llangybi, near Lampeter, where nursery numbers will have virtually doubled over a 12-month period by April.
The Assembly government is working on an action plan for developing foundation-stage staff, and has already announced funding for training an extra 450 Welsh-medium early-years workers. On-going monitoring and evaluation of the pilots will include staff training needs.