Bureaucracy is blighting trials of the Assembly government's widely lauded school effectiveness framework, heads will be told next week.
The framework was launched in 99 schools last year to test the best method of tackling school and teacher underperformance through the sharing of best practice.
But Cheryl Wheldon, head of Coedffranc Primary near Neath, is set to warn of excessive red tape in the system at the National Association of Head Teachers' conference in Brighton.
Delegates from across the UK will debate the framework, which critics say is being used as a "sticking plaster" for everything that is wrong with Welsh education.
Ms Wheldon, NAHT Cymru's president this year, said she had already been told by heads trialling the cross-local authority consortium approach - one of two methods being tested - that there was too much paperwork involved and not enough face-to-face contact with colleagues at other schools to share good practice.
There were also concerns that schools would not have the time or money to share good practice if the framework was introduced nationally.
Anna Brychan, NAHT Cymru's director, said while the union broadly supported the principles of the framework, the motion was a "shot across the bows" to warn the government that it must get it right.
"There is a lot of uncertainty among heads about the future direction of the framework," she said. "We want this to succeed, but we don't want an extra layer of bureaucracy."
Two methods are being trialled: a cross-local authority approach in south-west and mid-Wales; while in south-east and north Wales, "superheads" are being used to impart knowledge to schools in other counties.
The pilot schemes were due to end this month, but the Assembly government has extended them until the end of term.
A spokesperson said: "The minister has agreed to take the school effectiveness framework pilot to the end of this academic year. This enables the associates and improvement facilitators, who are working with their respective local authority education improvement officers, a full academic year to work with their schools and to develop effective professional learning communities."