A study for Christian foodbank charity the Trussell Trust shows 40% of teachers don't think their students get enough food.
It's being blamed on the lack of school meals and breakfast clubs which are only available in term time.
The charity's Chairman Chris Mould told Premier: "We're absolutely sure that this will be related to low incomes and people finding it very hard to make ends meet.
"We know from all the evidence we get from our work with foodbanks and the people that use them that parents prioritise feeding their children whenever they can."
He said the issue was become very serious and needed government action.
"We have two problems in this nation," he said.
"We have a massive problem with obesity but on the other end of the scale we have a serious problem with malnutrition.
"An increasing number of people, children in particular, who aren't getting the right food and aren't getting enough food."