Jackie Fothergirll, 34, said her son Rowan Parkin, 13, paid £1.80 for the panini during his lunchbreak at Hinde House secondary school in Sheffield.
But he was left disappointed when he opened it to reveal a few slices of chicken and sa tiny bit of cheese wedged between seemingly untoasted buns.
Jackie said: “It's just not acceptable for our children to be eating this kind of food. The portion is absolutely ridiculous and it costs £1.80. With the cost and everything is going up, that is not sufficient at all."
Jackie, who has filed a complaint with the school and the caterer, said it was not the first time she had voiced concerns.
And she said that some kids could go hungry if they were relying on school meals as their only food for the day.
She added: "To pay that amount of money for a meal and that's all you're getting, that could be some child’s only meal of the day. If that's a child's only meal, the child's going to go hungry throughout the day.”
She said after her complaint was made public, the school reached out to the catering supplier, Mellors, who then issued an apology.
But Jackie claims an apology is not enough, adding: "The supplier said they would reimburse the parents, but I don't want reimbursement. I want this fixed because this is not the first time that this happened.
“We have been complaining to the school for a long time but nothing's been done.”
She said she and other parents have been asked to go to the school for a 'secret shopping experience' to sample the food themselves. But she said this would not solve the problem either.
She added: “I just feel like since they know I'm coming, they're going to do their best. I mean, this is not the first time...the children have had enough.
“What they need to do now is to up the portion sizes to match the price of the food or lower the food price to match the portion control.”
In a statement, the shcool said: “We have been made aware that there are numerous concerns being circulated regarding the school's catering provision provided by Mellors.
“Please be assured we will be addressing all concerns with the contractors as a matter of urgency and will keep you all updated.”
In response to a letter of complaint, Mellors director of operations Anne Kavanagh said: “I have been made aware of the complaint surrounding a panini purchased by one of your pupils.
“The quality standard represented by the photograph taken does not reflect the standards with which we operate.
“We are undertaking an internal investigation to find out what happened and to ensure that the usual high standard of food operation is adhered to.
“I would like to apologise for this incident but rest assured that we are reviewing our procedures to ensure a similar situation does not occur.”