Ministers offered free meals for children between the ages of four and seven, with a small surcharge from 7 p.m.
And the school union is furious that lunch costs between £2.34 and £2.41.
It would have to be at least 23p if the increase is meant to prevent 10% inflation.
James Bowen of the National Union of Teachers said: “An extra £7 a day is not enough to cover the increased cost of schools.
"Funding is lacking and it is very difficult for schools and catering to provide healthy and balanced meals."
And Avril Chambers of the GMB union, which represents 100,000 school workers, added: “In a decade of Conservative rule, funding for education has come to a screeching halt.
Painting:
PA)It is a national shame that we are losing students. A 7p rise in the minimum cost of living for a generation will change nothing."
Labour's Rosena Allin-Khan addresses ministers after voter complaints in London.
He said: “Schools and councils are under huge pressure to help families when the government needs to step in.
A government spokesman said: "We know that many families are facing high costs, so the government is continuing to work to overcome the pressure."
All 1.25 million children receive free lunch at the front desk and ages 1 and 2.
But critics say the funding has actually been cut by £35, meaning schools have to draw more from their own budgets to pay for textbooks and teachers.
Liberal Democrat education spokeswoman Munira Wilson said: “This Government will hold our children to account.
"Ministers must provide the extra funding that our children so desperately need."
Every child should have access to two hot meals a day at school, which include a meat or vegetarian meal, potatoes, pasta or rice, salad and dessert.
