Small rural schools will struggle to cope if they are given more money and responsibility by their local authorities, teachers have claimed.
Members of Welsh-medium union UCAC have warned that a pledge by local authorities to stop holding back funding and move more cash to the classroom could be "onerous" for some heads.
Last year average delegation rates across Wales fell to a record low of 74.8 per cent, with more than #163;600 million earmarked for education retained centrally by local authorities.
Wales's 22 councils have since vowed to increase delegation rates to 80 per cent within two years and to 85 per cent within four years in the face of heavy criticism from the teaching profession and education minister Leighton Andrews.
In a motion that was unanimously agreed by UCAC members at their annual conference in Llandrindod Wells last weekend, Rolant Wynne, head of Ysgol Gynradd Dolbadarn in Llanberis, Gwynedd, called on the Assembly government to cap rates at 85 per cent.
He said: "It looks appealing on the surface, but it is very dangerous. The more monies distributed to schools, the more responsibilities there will be. It is going to be very onerous on heads of small rural schools if they have that extra responsibility, especially if they have to fund special needs provision and transport out of that.
"I think this money should be set aside for special needs education and transport. If the larger schools withdraw from these central contracts then the burden will fall on the smaller schools, and they won't be able to cope with the expense or administrative burden.
"We want consistency and transparency across authorities, but we think 85 per cent is sufficient. If 90 per cent is distributed it will pose a genuine threat to the education system in Wales."
The Welsh Local Government Association has said that decisions about funding must take into account local circumstances and that it "makes sense" for some services to be provided by local authorities rather than schools.