The caterers working at St Peter's Collegiate School in Compton, Wolverhampton, were sent home after a worker tested positive for Covid.
Principal Jon Arnold said no pupils had come into contact with the caterers, provided to the school by Aspens, and that the kitchen had undergone a deep clean.
Parents have been sent letters informing them of the development.
Mr Arnold said in the letter, dated October 9: "I have been informed by the school caterers, Aspens, that one of their team within school has tested positive for coronavirus.
"In accordance with Public Health England's advice, all of the remaining team members are now self-isolating as a precautionary measure.
"I would like to reassure you that at no time has any student come into contact with the school caterers, and that throughout the whole term they have undertaken and complied with the strict Covid-19 hygiene guidance when preparing all food bags for students.
"The entire kitchen preparation areas have been thoroughly deep cleaned and I now have a team of relief caterers in the school."