Four fire engines were called to the Lancaster Royal Grammar School at around 7pm on Tuesday May 22 following reports of a fire.
Firefighters say the fire involved a deep fat fryer but praised the swift actions of staff at the school who ensured the safety of pupils during the incident.
Scott Lee, Crew Manager at Lancaster fire station said: "Staff were serving the evening meal to boarders when the fire broke out.
"Unfortunately it was just one of those things. It seems some fat splashed from the fryer and caught light.
"We used dry powder extinguishers and a hose reel to put the fire out. Thankfully nobody was injured.
"Staff acted swiftly to evacuate the premises and to alert the fire service."
Crews spent around an hour at the scene.
Headmaster Dr Chris Pyle said: "One of our deep fryers caught light yesterday evening.
"Catering staff called the fire brigade, who brought the fire under control without any significant wider damage to the kitchen.
"All staff were evacuated safely, and pupils were not affected.
"I am very grateful to the fire brigade, and to the catering staff who have worked so hard this morning to clear up all signs of the smoke and damage.
"The kitchen and dining hall are back in use today."