The Association of Teachers and Lecturers (ATL) Cymru said it was nothing short of a national scandal that 150,000 children in Wales are now living in poverty - up by almost 6,000 on 2007.
But despite schools helping out (one had even funded exam kits for poor pupils) they were fighting a losing battle trying to erase child poverty and raise the educational aspirations of the most deprived, the association said.
Almost three-quarters of all ATL members in Wales say poverty affects pupil performance.
As part of the inquiry, the union is now recommending a change in tactics so that social class, rather than individual cases, are focused on in Wales. Attention to poverty-stricken women was particularly important, according to the ATL, which says too much responsibility is placed on the poor to "lift themselves out of the poverty cycle".
The ATL Cymru said in its evidence: "There should be a fresh debate on whether every part of our education system can promote social mobility if the Assembly government is serious about raising educational attainment."
The NASUWT Cymru also contributed, saying that underfunded schools could not be held responsible for attacking poverty alone. The union also said a blame culture between local authorities and the Assembly government often meant schemes failed.
Education minister Jane Hutt has given evidence previously to the cross-party committee for children and young people.
She told Assembly members that that new initiatives, such as Flying Start, were helping to tackle poverty in the early years.