The report, which came out in the Times Education Supplement (TES), says a 'sandwich tax' is being charged to pay for cleaning and supervision in lunch areas.
While a spokesman for the Department of Education told the TES the practice was 'absolutely unacceptable', the union said they expect to see more schools doing the same to cope with budget cuts.
Condemning the practice, the union's deputy general secretary, Patrick Roach, told TES: 'Now just sitting in a dining hall and unwrapping your sandwiches is considered to be an optional extra, it's disgraceful, it's shocking.
'Parents should be appalled in just the same way that we're appalled.'
Just last month, official papers revealed that parents in South Wales could be be forced to pay the controversial fee to Caerphilly council.
Plaid's Assembly candidate and councilor Lindsay Whittle warned all parents that if the council plans are approved, the charge wouldn't be taken on by the schools who are already short of cash but simply passed on to them which 'is not on'.
Lindsay said at the time: 'School budgets are already under pressure, so it seems likely that this charge will be passed on to parents by many schools.'
Alternatively, schools may just stop allowing children to bring their own lunches and instead force them to eat food provided for them in the school canteen.
Linsday explained how this would also increase cost for families who use packed lunches as an alternative to expensive school food; 'Some parents, although working, do not qualify for free school meals for their children so give them sandwiches because they can't afford to pay for meals.'
Not only would this cost parents more, but as many children refuse to eat school meals because they don't like them, the new rules could cause even more arguments for stressed-out parents.
Although Caerphilly council publically said they don't intend for parents to pay the extra tax, documentation revealed that ultimately it would be the individual school's decision, not the council's.
'Schools may choose to make their own arrangements for setting out and clearing away sandwich places, in which case the catering service will be able to reduce staffing hours and still realise the saving identified.'
If the decision is left to each individual headmaster, it is hard to tell whether or not parents will be left out of pocket.