Malnourished pupils with grey skin are "filling their pockets" with food from school canteens in poor areas due to poverty, head teachers say.
The heads, from various parts of England and Wales, described differences in the appearance of some pupils.
One head said: "My children have grey skin, poor teeth, poor hair; they are thinner."
The government said measures were in place to tackle poverty.
Lynn, a head teacher from a former industrial town in Cumbria, who did not want to give her full name, was one of a number of head teachers speaking to reporters at the National Education Union conference in Brighton.
They were highlighting the issues faced by an increasing number of children growing up in poverty, and how their experiences affect their education.
Lynn said that hunger was particularly apparent after the weekend.
She said: "Children are filling their pockets with food. In some establishments that would be called stealing. We call it survival."
Another head teacher from Nottinghamshire, Louise Regan, said: "When you take children out to an event, maybe a sporting event, you see children of the same age from schools in an affluent area.
"It's the grey skin, the pallor. It's the pallor you really notice."
She went on: "Monday morning is the worst.
"There are a number of families that we target that we know are going to be coming into school hungry.
"By the time it's 9.30am they are tired."
She said her school supplied some pupils with clean uniforms, and that they often came back in the same clothes, grubby, after the weekend.
The school has a food bank which gives out food parcels and a supply of clothes, shoes and coats for those without.
Poverty and neglect
Lynn said: "We have washing machines and we are washing the children's clothes while they do PE.
"We wouldn't have it that these children are stigmatised because their clothes are dirty."
The school also runs a summer school for three weeks over the holidays, run voluntarily by teaching staff without pay.
Howard Payne, a head at an inner city school in Portsmouth, said there had been a four-fold increase in the number of children with child protection issues.
"Every one of these issues has had something to do with the poverty that they live in," he said.
"It's neglect. It's because they and their families don't have enough money to provide food, heating or even bedding."
Mr Payne, who provides debt counselling and family support at his school, said: "Three weeks ago, many schools in our area closed because of the snow.
"I kept ours open because I was really worried about the children - that they wouldn't have a hot meal to eat that day."
He said about 45% of pupils came into the school to eat that day.
All the heads said things were getting worse as social and emotional support services were disappearing.
The comments came as the NEU published research it had carried out with the Child Poverty Action Group.
It found schools were increasingly stepping in to fill the poverty gap, with almost half of the 900 respondents saying their school offered one or more anti-poverty services such as a food bank, clothes bank or even offering emergency loans to families.
More than four-fifths said they saw signs of children being hungry during the day, and about the same said they saw children showing signs of poor health.
Alison Garnham, chief executive of the Child Poverty Action Group, said: "With nine children in every classroom of 30 falling below the official poverty line, it is time to rebuild the safety net for struggling families."
Lynn added: "My families are proud. Some of these parents are working two or three jobs and can't access the benefits system.
"They are just a few pounds over - they have less money than those on benefits."
Jane Jenkins, a head teacher from Cardiff, said children in her school often only brought a slice of bread and margarine for lunch and that teachers supplemented this.
"It's really difficult and when people are asking you about standards, why we don't go up the league tables?
"That's often a secondary consideration."
The Department for Education said it wanted to create a country where everyone could go as far as their talents could take them.
"That's why we launched our social mobility action plan, which sets out measures to close the attainment gap between disadvantaged students and their peers, and targets areas that need the most support through the £72m Opportunity Areas programme."
A spokesman also highlighted the £2.5bn it invests in disadvantaged pupils through the Pupil Premium and a recent £26m investment in breakfast clubs.