Proposals to make headteachers set test targets for each other will backfire because they will be too soft, education officers fear.
One official said that heads were so weak when it comes to challenging each other that it would be like "duelling with dry tea towels at 20 feet".
His comments were sparked by government plans to give every headteacher a "critical friend" - in most cases a former or current head who would help to set their performance targets and co-ordinate their funding.
The plan, which the Department for Education and Skills calls the Single Conversation, has received backing from headteacher unions.
But the Confederation of Education Service Managers (Confed) expressed concerns about the initiative to Peter Housden, DfES director-general of schools, at its annual winter conference in Birmingham.
Steve Munby, Knowsley's education director, said projects conducted in his authority in the past indicated that heads were much better at encouraging each other than being critical.
"Headteachers are excellent at the support and the encouragement, but they are less good at the challenge," he said.
"How can we be sure the challenge will be there?"
Mr Housden said he hoped that ways to ensure rigour would be found in the pilot projects, due to start in at least five local education authorities this year. He said it would reflect badly on the DfES and local education authorities if standards were not maintained.
"We've got a stake in this, so we have to be sure they are critical and challenging as well as supportive," he said.
The Secondary Heads Association, which has claimed credit for suggesting the scheme, denied that headteachers would be soft on each other.
Anne Welsh, president of SHA, said: "Headteachers are perfectly capable of being rigorous. If we are going to help and support each other, then we have to be challenging."
Mrs Welsh added that care was needed to ensure that the Single Conversation system did not add excessively to current headteachers' workloads.
The scheme was one of the biggest bones of contention at the Confed conference.
Education officers also quizzed DfES officials about the planned Children's Bill and how to reconcile moves to promote inclusion in schools with the competition created by league tables.
The Children's Bill, which is due to be debated in Parliament this spring, is expected to propose a children's commissioner and plans to make different local services work in greater collaboration.
Tom Jeffery, DfES director general for children and families, said there would be minimal legislation, but the Bill would be accompanied by a detailed paper currently called "The Way Forward".
He said the paper would help to contextualise the Bill and explain "a new paradigm of children's services with schools at their heart".