The NASUWT has signed a deal with the Co-op to back their schools - including academies.
The classroom union announced this week that it will promote the co-op model of school management as an alternative to them becoming academies run by private sector organisations.
The union said that where schools were intending to convert, it would press them to use the co-operative model to retain "public and community accountability".
Come September, the co-op will run more than 350 schools, including an estimated 20 academies.
Dave Boston, chief executive of the Schools Co-operative Society, said: "Co-operatives and trade unions come from the same background, traditions, shared beliefs and values.
"Our two organisations will work together to ensure that schools serve the best interests of children and young people, parents and carers, the workforce and the wider community."
And the NASUWT's leadership agreed. "This positive agreement represents an alternative to the privatisation agenda that sets school against school and puts parents and pupils at the mercy of the free market," general secretary Chris Keates said.
"Collaboration and co-operation is in the best interests of all children and young people."
For more on this story, see this week's TES out on Friday 20 July