The National Union of Schoolmasters Union of Women Teachers, which led a one-day strike in Newham, east London, over the same issue last summer, said teachers' conditions of service are at risk.
The union's members fear the proposed August start to the autumn term could leave the back door open for moves to a five-term year.
But Graham Lane, chairman of Newham's education committee, said teachers were still getting the same number of holidays, and accused them of "conservatism".
Last year's return date of August 31 led to an 84 per cent ballot in favour of strike action by NASUWT members. They walked out for the day - which had been scheduled as a training day - and had their pay docked. The union is considering taking the authority to court over the deductions.
This year, the new term is scheduled to start on August 29.
The union is hoping a settlement can be reached with the authority, but expects strong grassroots support for another strike if talks fail. If that happens, pupils, unaffected last year, will be hit.
Christine yer, NASUWT local branch secretary, said: "Once you establish the principle of working in August, you could end up going back on August 1. It's also traditional and psychological, teachers don't want to come back before September 1."
Derek Moore, the union's national executive member for north-east London, said the dispute was to do with conditions of service and holiday entitlement. He wants a settlement, but says feelings are running high among members.
He added: "There are far more pressing issues than a squabble over the school year, but it's important to our members. Tea-chers strongly feel that the authority is acting unreasonably."
But Mr Lane insisted teachers were getting the same holiday entitlement, and dismissed concerns about five-term years.
"The fact (the start date) happens to fall in August in some years and in September in others is irrelevant. Is there some principle that teachers don't want to work in August?" he said.
"We have taken away none of their holidays, they are doing 190 school days and five teacher days.
"Sometimes they do a couple in August, sometimes a couple in September. If they do a couple in August, they do less in July."