Headteachers are set to raise concerns that test and exam results can be manipulated by politicians to suit their own agenda.
There must be no possibility that governments and public bodies can influence grade boundaries to support their own policies, the National Association of Head Teachers (NAHT) is expected to warn.
Delegates at the union's annual conference in Telford are due to debate a motion calling on its executive to "expose and discredit the statistical and politically-motivated manipulation of test and grade boundaries" and highlight the "harmful and unjust implications" this can have of students and teachers.
Speaking ahead of the resolution, proposer Graham Frost, a headteacher from Carlisle, said: "We want to remove any possibility of government agencies manipulating school performance data to support a particular political agenda."
He said that his own research shows a steady rise in primary school SATs results over time, but small irregularities appearing around the time of general elections.
"What I'm contending is that I suspect there is a tendency to be tougher or less tougher in where they set those grade boundaries depending on the agenda," he said.
Fellow Carlisle headteacher Clem Coady said there are concerns that boundaries are changed to suit a particularly narrative.
"You see the boundaries changing year on year depending on what stories they want to do."
Labour said that politicians need to listen to teaching professionals rather than "setting grade boundaries for short term political gain".
A spokesman said: "Parents deserve to know exactly how their child is performing at school but the chaos this Government has caused by continuously chopping and changing grade boundaries is undermining confidence in results."
Liberal Democrat education spokesman John Pugh said: "Changes to the curriculum and the way qualifications are awarded should be decided by independent experts in the education field, not by politicians.
"We want to put an end to ministers meddling excessively in the way our children are taught and give pupils and teachers back some stability."