The accident caused an evacuation of the school.
Authorities reported that the science teacher and a student suffered third-degree burns to the head, face, arms and hands, while another student suffered burns to his arms and hands.
"These were not life-threatening burns but were very serious burns," said Federal Way Fire Department spokeswoman Monica Colby.
School officials and authorities did not release any names or conditions of those involved.
The teacher and the student who suffered "significant burns" were taken to Harborview Medical Center in Seattle.
The accident occurred about 1:30 p.m. while the science teacher was demonstrating how different metals and chemicals produce multicolored flames when mixed with methanol, school and fire officials said.
"This was an experiment in a science book, so this is routine. She had been doing it (this experiment) all day in other classes," said district spokeswoman Diane Turner.
But the teacher didn't realize the methanol in one beaker had already been ignited, because burning methanol produces nearly invisible flames, Colby said.
When she poured more methanol into the container, there was an explosion and fire quickly spread toward the first two rows of desks, authorities said.
Seeing his teacher's hair and face afire, one student threw his coat over her, authorities said.
"He may have saved her life," Colby said. The teacher suffered serious burns, but the burns would have been much more severe if the student hadn't rushed to her aid, she said.
Students also used three extinguishers to fight the fire, and by the time the Fire Department arrived, the fire was out, Colby said.
Another burn victim and a student who complained of chest pains and shortness of breath during the school evacuation were both treated and released from St. Francis Hospital in Federal Way, authorities said.
Authorities said the fire was contained inside the laboratory and only garbage cans, chairs and textbooks were damaged.
"The superintendent has directed we investigate the experiment, investigate what happened, what went wrong," Turner said.
Tan Vinh: 206-515-5656 or email@example.com