A small fire broke out at Longdean School on Wednesday morning during a cookery class.
The fire took place in a classroom adjacent to the school canteen so the decision was made by headteacher Graham Cunningham to send all students home that day.
Six sprinklers helped to quickly put the fire out at the school in Peascroft Road.
Mr Cunningham said: “Yesterday morning a small fire broke out in one of the technology classrooms. This activated the sprinkler system, which extinguished the fire within minutes. We acted immediately to evacuate students and staff in a calm and orderly way within 8 minutes.
“The decision was taken to close the school in order to clear the surface water created by the sprinklers and to ensure the school was safe. We immediately informed parents and carers and followed this up with a text and letter yesterday evening.
“Fire officers attended to inspect the damage and to confirm the school was safe to reopen. Longdean School is open today as normal.”
A Herts Fire and Rescue spokesman said: “At 11:45am we were alerted about the fire at Longdean School. One fire pump along with an officer were sent to the scene within minutes.”