Andrea Bradley, general secretary of the EIS union, said that, in the middle of a cost-of-living crisis, the move “runs contrary” to efforts to tackle child poverty.
She also criticised ministers for having “obscured” the position within the draft 2023/24 Scottish Budget.
The Scottish Government had previously promised to introduce free school meals for all primary school pupils by August 2022.
So far this has only been delivered for youngsters in P1 to P5, with First Minister Nicola Sturgeon saying in September that ministers would “start work with local authorities to extend universal provision to all pupils in primary 6 and 7”.
But the EIS said details in the Scottish Budget show the universal rollout of free school meals for P6 and P7 will now be delayed until 2024.
Ms Bradley said: “Delaying the rollout of free school meals to all primary school children was a shameful decision, which runs contrary to the Scottish Government’s stated commitment to tackling child poverty.”
She added: “This is now the second time that the rollout of this hugely important policy has been delayed, with serious consequences for thousands of children and families across Scotland.
“It is also extremely disappointing how this change in policy came to light – not announced in Parliament, but obscured within the detail of the Budget document itself.
“In a country where more than one child in four lives in poverty, and with the cost-of-living crisis pushing ever more families into financial difficulty, it is more important than ever that universal free school meals should be a priority.”
Ms Bradley said, while a “watered-down, means-tested policy” on free school meals was being introduced for youngsters in the final two years of primary school, many children could still miss out, “placing great strain on families already struggling with the soaring cost of living”.
“Universal free meals remain the best way to ensure that all young people have access to a healthy and nutritious meal at school, without any stigma being attached,” she said.
“The Scottish Government claims that practical barriers to universal rollout are the problem. The EIS view is that young people should not be hungry, stigmatised or left out whilst adults dither over dining chairs, tables and cutlery.”
Education secretary Shirley-Anne Somerville said, however, that Scotland had the “most generous universal provision” of free school lunches in the UK.
“All pupils in primaries 1 to 5, children in funded early learning and childcare, and eligible pupils in Primary 6 through to S6 benefit from free meals in Scotland – the most generous universal provision of anywhere in the UK,” she said.
“We will go further. Our additional investment announced in the 2023/24 Budget will continue to fund the expansion of free school meals for all Primary 6 and 7 pupils in receipt of the Scottish Child Payment, helping children most in need first.
“This will be the next step in fulfilling our commitment to universal provision in primary schools.”