Prime Minister John Key has rejected the idea.
"Not every school wants every child to be provided a lunch," he told media at the APEC conference in Vladivostok.
"There are many families that can provide those lunches."
But Mr Shearer said the Government had to "get real".
"If kids can't get to school and be ready to actually learn then they're not going to learn. Fifty per cent of our prison population are functionally illiterate so we've got to really make an effort to make sure that those kids don't slip behind.
"Providing food means that kids get a real good start but it also means that they're not distracting other kids in the class as well and that's really important."
Labour's policy also included extending the reading recovery programme to all New Zealand schools and increasing the number of six-year-olds who did it by 5000 a year - a change estimated to cost an extra $20m annually.
He said the programme, designed to significantly reduce the number of children with literacy difficulties, was "gold standard" with a success rate of more than 80 per cent, but was not universally available - only 59 per cent of low decile schools had it compared to 73 per cent of high decile schools.
Associate Education Minister Pita Sharples said he welcomed interest and support from across the political spectrum, but suggested the Reading Together programme was the best option as Reading Recovery was too costly.
The Government allocated $6.5 million in last year's Budget to expand family-based literacy programmes to all decile 1 to 3 schools, building on the Reading Together programme.
"Reading together looks at reading as a family issue and it gets great results because whanau and schools work together," said Dr Sharples.