Thirty-five-year-old Brian Clawson was mixing potassium nitrate with sugar when it exploded, catching his shirt collar on fire and burning his hands, face and arms Wednesday at Derry Area High School. The school is about 40 miles east of Pittsburgh.
One student standing nearby suffered a minor burn on his leg, and another girl fell while the school was being evacuated just before noon Wednesday.
Students say Clawson was primarily concerned about the safety of his students. Another teacher rushed to help him pat out the flames on his clothes.
School and federal officials will investigate what went wrong with the experiment, which Clawson performed in earlier classes without incident.