A St John spokesman confirmed it was "one patient deceased on scene".
He did not have further details on the person's gender, age, or whether they had any connection to the school.
Earlier a Herald photographer said he saw people gathering around an ambulance at the scene.
"There were about six police officers, an ambulance and half a dozen other people."
He said it looked like a blessing was being conducted, as the group had their heads bowed.
"They were in a little car park area by the sports field."
Kelston Boys High School principal Brian Evans was on his way to the school when the Herald rang him.
"I know absolutely nothing about it. I'm just going there now, I saw it on your website."
He said there were no sports matches scheduled for today.
"The school's open on Sunday morning, there is a church that uses the school, so the school was open but I'll go and try find out myself what's happened."