Many pupils living in poverty come to school hungry, tired and in worn-out clothes, a survey by the ATL teachers' union has suggested.
More than three-quarters of 627 primary, secondary and college teachers in England, Wales and Northern Ireland who responded to the survey believed they taught pupils living in poverty.
And about 40% said the problem had increased since the recession.
The government said it was targeting investment at the poorest families.
More than 85% of the teachers who responded to the survey said they believed that poverty had a negative impact on the well-being of pupils they taught.
'Not eaten for three days'
Of those, 80% said students came to school tired, 73% said they arrived hungry and 67% said they wore worn-out clothes or lacked the proper uniform.
Also, 71% said pupils living in poverty lacked confidence, and 65% said they missed out on activities outside school such as music, sports or going to the cinema.
It total, 80% of teachers surveyed said they believed poverty had a negative impact on pupils' attainment - with problems including under-achievement, not having a quiet place to study at home, inability to concentrate and lack of access to computers and the internet.
"Every day I become aware of a child suffering due to poverty. Today I have had to contact parents because a child has infected toes due to feet squashed into shoes way too small," a teaching assistant in a West Midlands secondary school told researchers.
A Nottingham sixth form teacher said one pupil "had not eaten for three days as their mother had no money at all until pay day", while another teacher said a boy had come to school with no underpants and been laughed at by peers while changing for PE.
Anne Pegum, a further education teacher in Hertfordshire, said: "We have students who miss out on meals because they do not have money to pay for them and in some cases then feel unwell and have to be helped by our first aiders."
And secondary school teacher Craig Macartney, from Suffolk, said he had noted that increasing numbers of children from middle to lower income households were missing school trips as families struggled to meet the basic cost of living.
'Investment for the poorest'
ATL's general secretary, Mary Bousted, said: "It is appalling that in 2011 so many children in the UK are severely disadvantaged by their circumstances and fail to achieve their potential."
"What message does this government think it is sending young people when it is cutting funding for Sure Start centres, cutting the EMA [Education Maintenance Allowance grant for low-income students], raising tuition fees and making it harder for local authorities to provide health and social services?", she asked.
A spokesman for the Department for Education said the government was "overhauling the welfare and schools systems precisely to tackle entrenched worklessness, family breakdown, low educational achievement and financial insecurity".
"We're targeting investment directly at the poorest families. The most disadvantaged two year olds will get 15 hours free child care. We're focusing Sure Start at the poorest families, with 4,200 extra health visitors.
"We're opening academies in areas failed educationally for generations and bringing in the pupil premium to target an extra £2.5bn a year directly at students that need the most support," the spokesman said.
Earlier this month, the government said changes to the benefits system would lift 350,000 children out of poverty, as it published its child poverty strategy.
The shift to the universal credit system would enable people to work themselves and their families out of poverty, ministers said.
But the Institute for Fiscal Studies predicted in December that the welfare shake-up would increase relative child poverty figures by 200,000 in 2012-13 and 2013-14.
And anti-poverty campaigners say the government's approach is wrong, as many families living in poverty have someone working full time on low wages.
They are also concerned that poor families with young children will be adversely affected as councils facing budget cuts reduce Sure Start services.