The prime minister has been told he would be "crazy" to move Ed Balls from his post as Schools Secretary ahead of an expected Cabinet reshuffle this summer.
There has been speculation in recent weeks that Mr Balls could be moved from the Department for Children, Schools and Families to take over from Lord Mandelson as business secretary.
In an open letter to Gordon Brown this week, the Association for School and College Leaders warned that Mr Balls has a lot of "unfinished business" in education. The letter also claimed it would not be "good government" to introduce another education secretary with more policies when schools are just getting to grips with current legislation and guidance.
In the letter, John Dunford, general secretary of the ASCL, points to Labour's "contradictory" schools policy being a result of there having been six education secretaries in 12 years of government.
Dr Dunford writes: "Each secretary of state comes in with a shiny new policy programme and, given that they can only expect a brief sojourn at education, they want it implemented in a hurry.
"Meanwhile, schools and colleges are still implementing the hastily legislated ideas from the last two secretaries of state.
"With a maximum of a year before the general election, it's not a good time to take on a department as big as the DCSF, with a complex agenda that only a brain the size of a small planet can be expected to understand fully in that time."
The letter also states that with one education bill being debated and another expected to be introduced in Parliament next month, Mr Balls should remain to oversee the passage of his own handiwork.
"We will have a mad rush for a new version of the white paper before the summer recess and legislation cobbled together for the Queen's Speech in the autumn. Not good government, I am sure you will agree," Dr Dunford adds.
The ASCL admitted the letter would have little effect on the Prime Minister's decisions, but said it was written to remind people of the effects that a change in minister can have on schools.
The idea was disparaged by other teachers' unions, who said the ASCL was "whistling in the wind" if it thought it would have any bearing on the reshuffle.
Letters, pages 22-23.