Nigel Fredericks, the butcher that supplied Chartwells with halal chicken sausages found to contain pork DNA, has described it as an "isolated incident".
The discovery was made after the local authority carried out its own testing in the wake of the horse meat scandal.
Nigel Fredericks said the product was sourced from Brook Farm Sausages before being supplied to the school via a third party distributor.
Nigel Tottman, managing director of Nigel Fredericks, said in a statement: "We have used Brook Farm Sausage for many years, and can only think that this was an isolated incident involving some element of human error."
Testing on more than 15 samples of Nigel Fredericks products has been carried out since media speculation began in January concerning the integrity of meat labelling, the company said. Most were done at random and without the butcher's knowledge.
"So far, with the exclusion of the sausages named in this incident, every result that has come back has been certified as completely clean from anything other than what was stated on the label," said Tottman.
"We are continuing to send more products for testing, to help satisfy all customers and potential end users that our product traceability and safety is robust."
The company is now working with Chartwells and Brook Farm to investigate the matter.
"We are very sorry that we have had a product come through our supply chain that has failed to meet the high standards we and our customers expect, and extend our apologies to any person who has potentially eaten this product."