Brune Park Community School in Military Road was badly damaged by the fire, which broke out shortly before 5.53pm.
More than 40 firefighters from across the county were mobilized to battle the blaze, which spread through a recently closed building in Brune Park.
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Crews from Gosport, Fareham, Havant, Portchester, Southsea, Wickham, Hightown and Eastleigh fought for nearly three hours to bring the blaze under control.
In total, emergency personnel used 17 sets of breathing apparatus and two main jets to finally extinguish the flames, which were believed to have ignited on a flat roof on the first floor.
A spokeswoman for the Hampshire and Isle of Wight Fire and Rescue Service said around 10 per cent of the building was destroyed.
Officers successfully extinguished the fire at 9:25 p.m.
Dozens of people outside school boundaries watched as the flames died down.
Among them was 7th grader Ella-Mai Mcintosh-Sayles of Rower.
Talk to The news Courtesy of her mum, the 12-year-old said: ‘I just couldn’t believe it, it was actually pretty crazy to see my own school on fire. The entire building was surrounded by firefighters.
“There were seven fire engines and four police cars. We saw that the building was on fire and flames could come out of the roof, walls and windows.
“I could see firefighters going in and out of a broken window making sure no one was in the building.”
The cause of the fire is not yet known, but a Hampshire Police spokesperson said they were investigating and treating the incident as suspicious.
Firefighters re-inspected the abandoned building at 7 a.m., with no further action taken.
The school is due to reopen on Monday, and a spokesperson for Gosport and Fareham Multi Academy Trust said this was due to the rapid response from firefighters.
He added: “We would like to say a big thank you to the emergency services who responded to the fire at Brune Park Community School.
“It was their quick action that brought the fire under control quickly, preventing a more serious situation and further damage to the building.”