Michelle Telford called fire crews to the scene at Saltergate Primary School early on Sunday morning, August 12.
Michelle said she was driving along Skipton Road when she noticed the smoke.
She said: "I was driving down the A59, I saw the smoke and knew there was only houses and the school along there.
"I pulled up and saw it was the outdoor summer hut on fire, so rang the fire brigade as no-one was already at the scene.
Michelle added: "Unfortunately there had already been so many people driving and even walking past, looking at it, that hadn't even rung it in."
But fortunately within minutes of making the call, firefighters had arrived on scene to put out the blaze.
After Michelle uploaded videos of the fire to social media, residents across Harrogate have vowed to help the school by setting up a Just Giving Page to raise funds for the damage.
But more immediately, Harrogate cycling instructor and parent at the school, Dave Burns, has organised a mass clean up operation.
Mr Burns, who runs I Want To Ride My Bike cycling club at the primary school has used Facebook to gather troops to clear the area today, August 13.
He said: "I contacted the head teacher and offered to help sort it out. Hopefully we can get the site clear today and then we can take it from there."
Harrogate Police Officer, Amanda Hanusch-Moore took to Twitter to condemn the actions of those responsible for the fire, confirming that North Yorkshire Police are treating the incident as deliberate.
Mr Burns added: "It's just really sad, why would you do it at all but why would you do it at a school. Something needs to be done, but what and how, I'm not sure."