Fire crews were sent to The Minster School on Deansgate, which educates around 40 of the choristers at York Minster.A fire had broken out in the school kitchens at around 9.30am, but the blaze has now been contained.
The preparatory school, which has around 180 pupils aged 3-13, was evacuated during the incident.
Nobody is believed to have been injured and firefighters are ventilating the building.
The school has ancient origins, but was founded in its current guise in 1903. Its campus includes several listed Georgian and Victorian buildings, which stood in as the House of Lords in the 2016 ITV period drama Victoria.