The poorest children are going hungry due to "Dickensian" levels of poverty, as teachers fear huge swathes of the classroom are struggling to concentrate during the day due to missing meals.
Members of the Association of Teachers and Lecturers (ATL), meeting at their annual conference in Liverpool, reported anecdotal evidence of hunger among young learners.
It came as a study of around 400 ATL members found four in 10 (39%) know of pupils who come to school hungry and have no money to buy lunch, but who do not receive free school meals because they are slightly above the qualifying threshold.
Niamh Sweeney, a teacher in the Cambridgeshire and Peterborough area, said: "I am a modern history graduate and the levels of food poverty and hunger I see today reflect those from the early 20th century.
"In 2016 there are people for whom going hungry on a daily basis is now almost inevitable. You know you have children in your own school and class who come to school hungry and go home not knowing when they will next have another meal.
"And what happens in the holidays? Children and families get hungrier. What food they have is shared around and children ask before they eat 'is this lunch or dinner?' which means 'Am I going to get anything else?'"
Charles Gilmour, a teacher in a secondary academy in Surrey, said: "We have students who are entitled to free school meals whose parents do not claim because they do not wish to, due to social or work circumstances."
Concerns were also expressed about pupils suffering over the long summer break, with almost half (49%) saying they believed it negatively affects pupils' mental health.
Some staff said they brought cereals in for those who had come to school hungry.
Dr Mary Bousted, general secretary of the ATL, said: "The effect of hunger on pupils' learning is evident and it is shocking that in the 21st century so many pupils still come to school hungry with no means to buy lunch.
"With many families having to rely on charities such as food bank handouts, we risk returning to a Dickensian era rife with inequality.
"It is encouraging that this year's Budget has committed to offer £10 million funding a year to expand breakfast clubs in up to 1,600 schools, starting from September 2017, to ensure more children have a nutritious breakfast as a healthy start to their school day.
"However, this is just a drop in the ocean for the thousands of pupils who miss out on breakfast and aren't entitled to free school meals because they are from a low income working family."
Lindsay Graham, author of the report Holiday Hunger, Filling the Gap, said: "I find the results of this survey truly awful.
"It is 2016 and child hunger in the UK, at school or out of school, is unacceptable."
A Department for Education spokesman said: "As a result of continued investment by this Government, all infant pupils can get free school meals - meaning 1.3 million more children get a nutritious free meal at lunchtime, saving families hundreds of pounds.
"To ensure more children have a nutritious breakfast as a healthy start to their school day, we announced £10 million funding a year as part of the Budget to expand breakfast clubs in up to 1,600 schools starting from September 2017.
"Free school meals play a key role, giving children fuel for learning, and we continue to work with both councils and schools to encourage take-up."
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