Heads are being urged to “vigorously” oppose the expansion of grammar schools in England.
The National Association of Head Teachers will debate calls to reject a new wave of grammars in the absence of evidence that it aids social mobility.
Its conference will hear claims Conservative plans to expand selection will only add to pressures on schools.
Prime Minister Theresa May argues grammars boost opportunities for bright but disadvantaged children.
But Labour and the Liberal Democrats oppose the move.
The NAHT argues that investment in early-years education is a more effective way of ensuring that poor children get as good an education as their richer peers.
It says that by the time children leave primary school, the advantages of coming from a wealthy background are entrenched.
Proposing the motion at the NAHT conference in Telford, Robert Campbell said: “The government is fixated with delivering a policy for the few at the expense of the many.”
It would only add to the already intense pressures on schools, he said.
The union is using its conference to highlight what it describes as a “perfect storm” of difficulties for secondary schools this summer which could have “dire consequences for pupils and standards”.
Issues range from a new secondary exam system, struggles with recruitment or retention, and the £3bn real term cuts to schools budgets by 2020.
On funding, General Secretary Russell Hobby said there was a sense that the pleas of head teachers, teachers and parents over school funding were still not being heard by ministers.
He said: “All that’s left is for us to make it a general election issue.
“We need to make sure that all our parliamentary constituencies are asking questions on school funding.”
NAHT heads said schools were increasingly being required to fill the gaps left by under-funded public services, as finances become progressively stretched at local level.
Past president Kim Johnson said schools in his area, Medway, were being asked to part-fund police officers working in schools but they simply did not have the funds to do so.
He said out of the £6m budget for his special school, Bradfields Academy, he was paying £200,000 towards support on social care and health issues for his pupils.
This included two social workers, three therapists and counsellors, who were not being provided by frontline public services.
The government continues to insist it is funding schools at a record level.