Parents are being asked to hand over money to help schools pay for pens and paper as well as textbooks, trips and computers, a survey suggests.
In some cases, families have received requests for donations to help with the upkeep of school buildings, according to a poll by the Association of Teachers and Lecturers (ATL).
It also indicates that that many school staff are worried that pupils could be put at a disadvantage or made to feel left out if their parents are unable to donate money.
The findings show that more than two fifths of parents (43%) have contributed up to £50 a year per pupil in voluntary contributions for activities or goods that are not linked to their child's schoolwork.
And 70% have donated up to £50 a year per pupil to help pay for items and trips that are related to the school curriculum.
The poll, which questioned around 500 people working in England's state schools, comes as ATL is due to debate a motion on the issue at its annual conference in Manchester next week.
The resolution calls on the union to express concerns that schools increasingly need to ask parents for voluntary contributions and the effect it has, particularly on poorer families.
It says: "The burden of meeting this request falls disproportionately on poor families and increases the disadvantages for these children."
Jo Inglis, an ATL member from Wokingham who is proposing the motion, said that the poll had found that just 7% of teachers felt that asking parents for donations had no impact on disadvantaged pupils.
"Whether it is to be made to feel like an outsider, not be able to go on a trip linked to the curriculum, or not have access to revision materials; all make the often already difficult lives of these pupils even more difficult."
The poll found that over a quarter (26%) of staff say that their school or college asks parents and carers for voluntary contributions towards buying text books or revision materials.
Nine in 10 ask for money to help pay for school trips related to the curriculum, 78% ask for money for trips outside of the curriculum, about one in eight (13%) ask for help buying school stationery, such as pens and paper, and the same proportion ask for donations towards musical instruments and books.
The survey also found that about 5% ask for financial help towards the upkeep and maintenance of buildings, and similar proportions ask families to help pay for sports or IT equipment, such as computers.
Around eight in 10 (82%) of those surveyed said that their school or college makes up the difference if parents cannot give money towards curriculum-related trips, but just 30% said that the difference is made up for the purchase of text and revision books, and one in four (25%) said that their school or college makes up the difference for stationery.
About 43% said that they thought a pupil would be put at a disadvantage if their parents were unable to contribute towards resources and activities because the youngster would not have access to the things they need to study or revise, while a third (33%) thought it would make a student feel like an outsider if their family could not contribute.
Jo Fludder, a Reading teacher who is seconding the motion, said: "Parents who have difficulties putting their children into school uniform, providing them with transport to school and with food, find the added pressure of additional money demands from schools very stressful."
In total, 18% of education staff said they felt it is disproportionately unfair to ask poorer families for contributions.
ATL general secretary Dr Mary Bousted said that budgets are being squeezed more than ever, forcing schools to rely on parents to help pay for items that support their curriculum.
"Without voluntary contributions from parents and carers many students would miss out on a rich, diverse, school experience.
"However there is a big problem with hidden poverty, where some families feel too embarrassed or proud to ask for help, and more needs to be done to support those families so their children do not miss out on important learning activities."
:: The poll questioned 503 school leaders, teachers and support staff working in schools and colleges in England, Wales and Northern Ireland between March 17-24.
A Department for Education spokesman said: "While schools can ask for voluntary contributions for textbooks, musical instruments or other equipment, it must be made clear to parents that there is absolutely no obligation to donate.
"If a parent is unable or unwilling to pay, their child must be given an equal chance to take part in school life.
"We have protected the schools budget and just last month announced that we are giving an extra £350million to schools in historically under-funded areas".