The horsemeat DNA was discovered in cottage pies delivered to 47 schools in Lancashire.
It has now been removed from kitchens and officials say children would have consumed only a “minute” amount of horsemeat.
But many tests of products from some of the major school food suppliers have come back negative.
Analysts at one of six UK laboratories carrying out the tests discovered the horsemeat late on Thursday. Head teachers in the area have been alerted to the finding.
Lancashire county councillor Susie Charles, cabinet member for children and schools, said: “We share the concerns people have about what is clearly a major problem in food supplies across the UK and Europe.