Children are arriving at secondary school too hungry to learn, prompting teachers to spend their own money on food, a charity has warned.
Together Creating Communities (TCC) launched its all-Wales Stop School Hunger campaign at Ysgol y Grango in Wrexham on Monday.
It called for the free school meals allowance to be increased by 80p.
The Welsh Government said ensuring all our children fulfil their potential was a key priority.
The charity said many children were spending their allowance on breakfast which meant they could not afford a proper meal at lunchtime.
Head teacher of Ysgol y Grango, Stephen Garthwaite, said: "I have seen first-hand the way hunger is destroying our children's futures...
"Free school meals are no longer doing the job they are meant to be doing, with teachers bearing the brunt of its shortcomings and propping up a failing system."
Teaching assistant Yvonne Girvan, said she found it "impossible to turn a blind eye when a child arrives at school hungry".
"Most staff I know are doing the same, spending their own money on feeding children," she said.
TCC, which works in Wrexham, Flintshire and Denbighshire, said an additional 80p per child for the 29,223 children currently in Wales receiving free school meals would cost the Welsh Government about £3m.
In Wrexham the free school meal allowance is £2.45 but this varies between counties.
A Welsh Government spokesperson said: "Ensuring all our school children fulfil their potential is a key priority for this government and we have a number of policies in place to help ensure none of our school children go hungry.
"Unlike in England, all primary schools in Wales can access free breakfast provision for their pupils. We are considering proposals on how this important policy can be further extended and will be making an announcement in due course."