A survey by the Principals Federation has found nearly half of boards of trustees are worried about plans for national standards for primary school children.
Less than 3% of the 600 principals surveyed said their boards gave the policy their unanimous support, while 49% said they had either serious misgivings or some disquiet about the standards, the federation says.
The union's president, Ernie Buutveld, says the results echo a mid-year survey of principals, which found almost 90% opposed the scheme or had concerns.
He says a standardised assessment regime will shift the emphasis from teaching toward testing and reporting.
However, Education Minister Anne Tolley says the union is scaremongering. National standards will be used as a tool to identify children who are falling behind and to support quality teaching, she says.
Mrs Tolley accepts that many principals and teachers have concerns, but says federation has been spreading false information about national standards.