Two crews attended Hutton Grammar School on Liverpool Road at around 11.20am on July 26 after a member of staff alerted emergency services.
Firefighters arrived to find the fire had already been extinguished when they arrived although smoke was still present.
A spokesman for the fire service said: "The fire is believed to have been started accidentally and may have been caused by electrical equipment.
"Paramedics gave precautionary checks to three people."
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Damage to the school is believed to be minimal.
Ambulance services confirmed three adults were checked for smoke inhalation but nobody is believed to have been seriously injured.