Photos posted to the school's Facebook page show a pile of rubbish on fire and flames licking the side of the building with damage visible.
"It's not structural, but we will have to replace a panel on the shed."
She said on the other side of the wall was a gas bottle and toxic cleaning chemicals.
"At the time you just go into automatic.
"When I got home I was shaking a little bit and when I read the comments on Facebook I had a little cry."
The Facebook post said "fire-fighters (sic) tell me that if they had have been another two minutes later the whole shed would've been up in flames.
"A good reminder of why it is not safe for our students to be on school property after hours."
Ms Fitness said the community was saddened by what had happened.
"Mostly we are so grateful for how it turned out."