The pressures on such children, burdened with a responsibility that adults would find onerous, are enormous, and education inevitably suffers. Some young carers are still at primary school and may be looking after a disabled parent single-handed.
The association, which has just published a leaflet for schools in an attempt to raise awareness of the problem, said teachers could play a vital role in helping these children.
The leaflet points out that while some pupils may be identifiable by their frequent absences, others will strive to complete school work to avoid being identified as odd by their peer group.
Apparently many of them worry that if they complain or ask for help, the family will be split up and they will be taken into care. Disabled parents harbour the same fears, the association says.
The leaflet cites the case of a 12-year-old-girl who told classmates she went home to feed the dog at lunchtimes because she did not want them to know her mother had multiple sclerosis.
Teachers' main role is to give the children a chance to talk about their situation without fear of the consequences and direct them towards appropriate agencies.
The leaflet also offers practical suggestions such as making sure pupils can make private telephone calls home to check on their relative. This can also make taking a day off school unnecessary.
Sylvia Heal, young carers' officer at the association, said the size of the problem was difficult to estimate as there are no official statistics on the number of school-age carers, either from the Department of Health or local health authorities.
What is clear is that support for these families is often woefully inadequate. A survey last summer of more than 600 young carers found that a quarter were not getting any help at all.
Sylvia Heal said teachers could also be reluctant to get involved. "I spoke to one young university student who said she used to write essays at school describing her family situation, in which she was the sole carer of her mother. She got As and B-plusses for the essays, but no teacher ever questioned her about the content. They assumed it was fiction."
Teachers - young carers need you, is available from the Carers National Association which is based at 20-25 Glasshouse Yard, London EC1A 4JS. Tel: 0171-490 8818.