A head teacher has asked "how difficult is it to bake a potato?" in a despairing letter to parents about the state of food at his own school.
Jason Ashley, head of Redbridge Community School in Southampton, said meals served in its canteen were "completely unacceptable".
He blamed caterers Chartwells for the issues and said he had no control over what was on offer to pupils.
The firm has apologised and promised an "immediate action plan".
Mr Ashley's letter to parents, along with pictures showing some of the food, has been widely shared on social media.
"If my own children were served it I would be exceptionally unhappy," he wrote.
The head, who has also complained portion sizes have been getting smaller while prices were rising, said several meetings had been held with the private catering company to try to improve the situation.
"We have concluded with our photo evidence that Chartwells seem to be unable to bake a potato ... I keep asking myself, 'how difficult is it to bake a potato? - answers on a postcard'," he said.
Mr Ashley added he was "angry and disappointed" at the provision of meals and apologised to parents of the foundation secondary school, which caters for children aged 11 to 16.
"For many of our students, a school meal is so important to them but I can no longer tolerate, provide reasons or even justify to you the unacceptable nature of the food," he said.
Support services at the school are provided under a Private Finance Initiative (PFI) contract, with a management company determining the contracts for a range of activities including provision of food and meals.
"Ongoing catering issues" was one of the problems Mr Ashley raised at a meeting in November, discussing PFIs in Southampton schools.
Southampton City Council confirmed that schools within the city generally manage their own catering contracts, but those with PFI contracts have "little or no control over the quality of the services provided".
Chartwells said there had been "operational challenges" and staffing issues at the school.
In a statement, it said: "We apologise that in this instance our usual level of service has fallen below the high standards we demand.
"We are committed to working in collaboration with the school and are implementing an immediate action plan to rectify these issues."
It said its prices had not risen in the past two years, despite soaring inflation, and portion sizes had not been reduced and were in line with national standards.
On its website, the company says it is "the UK's leading catering company", providing daily meals for thousands of students.
It states it is "dedicated to delivering the nourishment to help fuel and inspire young minds".
