The teen's name has not yet been released. He is at Children's Hospital in Oakland where he is fighting for his life.
He was found at the high school, lying outside the gym. Police wonder if he was hurt while playing an extreme sport on the roof.
Santa Rosa police officers are talking to possible witnesses and looking for any evidence at the spot where a 15-year-old boy was found unconscious with severe head trauma.
"We're not sure what happened, if it was that he was the victim of felonious assault or if it was just an accident where he fell off the roof here while playing on the roof of the school," Santa Rosa police spokesperson Sgt. John Cregan said.
The fire department arrived to help police access the gym roof to look for any evidence of a fall.
Police say the teen also had cuts to his knuckles and scratches on his arms. He left the house on Saturday afternoon, telling his parents he was going for a walk. A short time later a passerby found him and called 911.
Police say the teen has been on the roof of Montgomery High before.
"Detectives discovered that several months ago this young man and several of his friends were actually here on the same campus and were engaged in parkour -- jumping from roof to roof and against walls," Sgt. Cregan said. "One of the young men fell and broke his leg."
YouTube videos posted by AmericanParkour, a training group which teaches the extreme and sometimes dangerous sport of urban acrobatics, shows what the extreme sport entails. It can include jumping from rooftops.
Parkour started in France and gained popularity when it was featured in a scene in the movie "Casino Royale."
Some students at Montgomery have heard about some trying the sport on campus, where midcentury flat rooftops make it possible.
"Adrenaline junkies just need it," student Shannon Connell said. "But I don't get it."
So far no witnesses have come forward.
The teen did not go to school at Montgomery. He attended another school across town.
Doctors hope to bring the teen out of an induced coma this week. Police hope to talk to him then.