Education Minister Hekia Parata has told the School Trustees Association to keep politics out of school newsletters.
Ms Parata gave the warning at the association's annual conference on Friday in Wellington.
A number of schools have sent out newsletters that included vocal opposition to some Government policies.
The minister's warning comes weeks after she dropped a plan to increase class sizes for some school years in the face of widespread opposition from schools and parents.
Some of that opposition was expressed in school newsletters, and Ms Parata says that's the wrong place for political agenda.
Green Party education spokesperson Catherine Delahunty says principals, teachers and boards of trustees have a right to speak out.
Ms Delahunty says anything that deters school communities from discussing education issues is unacceptable.
Secondary Principals' Association president Patrick Walsh says says Ms Parata's attempt to shut down political debate in school newsletters is verging on being anti-democratic.
Principals have a responsibility to inform parents of policies that affect the education sector, he says.
Punui Primary School board chairman Shane Haylock was at the conference and agrees with Ms Parata that it's wrong to use school newsletters for political arguments.
However, he says there is a place for necessary political debate.
Mr Haylock says schools can inform parents about policy change, but it must be done without being biased.