It is believed the fire started at the school, on Ainsdale Drive in Ashton-On-Ribble, Preston, shortly before 5.50pm.
Five crews from Preston and its surrounding areas remained at the scene at 7.45pm.
A Lancashire Fire and Rescue Service spokesman said: "Five fire engines and an aerial ladder platform have been called to a fire at a school on Ainsdale Drive, Preston. Firefighters are using two jets, two breathing apparatus and the aerial ladder platform to extinguish the fire. No injuries have been reported."
Amanda Metcalf, who lives opposite the school, said: "We were eating tea; we have patio doors and my garden over looks the school. My husband looked up saw that it was on fire and I rang the fire brigade.
"There was lots of smoke too, the wind was blowing it a lot so it filled the whole playing field and the back gardens."
A road closure has been put in place outside the school.
A spokesman for Lancashire police said: "There is a fire at the school and we are supporting our colleagues from Lancashire Fire and Rescue Service, assisting with road closures.
"There is a closure in place immediately outside the vicinity of the school where the fire engines are.
"It's too early to be speculating the cause of the fire because they (the firefighters) won't have had the chance to go inside yet."