Paramedics called to the school late Thursday morning had originally confirmed the male died at the shop at Ottawa's Mother Teresa Catholic High School.
They later said the student's vital signs were absent en route to the Ottawa Hospital's Civic campus but resuscitation efforts were successful and he regained a pulse. His injuries are believed to be life-threatening.
A 17-year-old boy was originally believed to be suffering serious injuries, but Ottawa paramedics spokesman J.P. Trottier later said the boy's injuries were not serious and said he is in stable condition at hospital.
At least three other students and one teacher were taken to hospital as a precaution.
A drum of oil in the shop is believed to have ignited, causing the blast, according to witnesses. It is not known exactly how the drum exploded, but students from the class said it contained peppermint oil, which is combustible when exposed to heat.
The school of about 1,400 was evacuated and the students transported to Longfields-Davidson Heights Secondary School.
Students and a teacher told CBC News that the school's public address system was down, and the order to evacuate had to be given verbally from class to class.
Ottawa fire services spokesman Marc Messier said there was no fire with the explosion and no severe structural damage.
Hazardous material crews are investigating and assessing the damage.