Keiron Simpson fell around 20ft (6m) from the top of the disused Habergham High School in Burnley at about 21:45 BST on Saturday, police said.
The teenager, who had been with with friends, was taken to Royal Manchester Children's Hospital suffering from a fractured skull and internal injuries.
Lancashire Police believes at this stage that it was a "tragic accident".
Det Insp Alisa Wilson said: "This young man has suffered serious injuries and while detectives are investigating the circumstances surrounding the fall, at this stage there is nothing to suggest this was anything other than a tragic accident."
Ruth England, the head teacher at Shuttleworth College in Padiham, where Kieron is a pupil, said it was a "very upsetting incident" which has "shocked all of us in the school".
"We have been in touch with Kieron's family to offer our support and let them know that we are all thinking of them and sending our very best wishes for a speedy recovery for Kieron."
A 14-year-old boy is in a critical condition after falling off the roof of a school in Lancashire.