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The agency, the United States Chemical Safety Board, distributed the video warning to its 60,000 subscribers, a spokeswoman, Hillary Cohen, said Friday, but it had no sure way to reach individual teachers at schools like Beacon High School on the Upper West Side. There on Thursday, Anna Poole, a young science teacher known for safety consciousness, used methanol as an accelerant to burn dishes of different minerals in the chemistry demonstration known as the Rainbow.

With about 30 students watching from their desks, a snakelike flame tore through the air, missing the students closest to the teacher's desk, but enveloping Alonzo Yanes, 16, searing and melting the skin on his face and body, according to witnesses. He was in critical condition on Friday in the burn unit of NewYork-Presbyterian Hospital/Weill Cornell Medical Center, Myrna Manners, a hospital spokeswoman, said.

Another student, Julia Saltonstall, 16, saw her thin T-shirt burned off her torso in an instant as some of her long dark hair went up in smoke, her father said. Though she was no farther from the demonstration than Alonzo, she escaped with only first-degree burns.

"She came so close to being so much more badly hurt," said her father, David Saltonstall, who had just started his first day as policy director for Scott M. Stringer, the New York City comptroller, when he got a 9:30 a.m. call from the school nurse that his daughter was in an ambulance heading for the hospital. His daughter called from the ambulance moments later, her voice quavering as she described the terrible accident.

"All of us feel just so terrible for Alonzo and the struggle he has ahead," Mr. Saltonstall said, describing the boy, a classmate of Julia's since middle school, as "funny and bright." His parents, who live in the Bronx and were holding vigil at the hospital, declined to speak about their son's condition.

The safety board's video, and an accompanying message, did not say the Rainbow demonstration should be banned, but warned that accidents have repeatedly occurred because of the volatile material involved.

"What we need to look at is why is this accident keeps happening across the country," said Mary Beth Mulcahy, a former high school science teacher who is now an investigator with the safety board, which has documented at least seven similar accidents, including a 2006 case featured in the video that left a 15-year-old girl in Ohio, Calais Weber, with severe burns over more than 48 percent of her body. "What do we need to do to stop the cycle?"

As a 23-year-old teacher, Dr. Mulcahy added, she herself did the rainbow demonstration, unaware of the potential dangers. The visually exciting demonstration shows how different substances produce flames of different colors because of their varying properties. But she added, "I can't imagine a teacher would do this demonstration if they knew the potential risk they were putting students in."

Said Ms. Weber, now a pre-med student in Boston: "I read this article last night about the New York students and honestly, I cried. I can't believe this keeps happening."

Other cases when the demonstration caused an explosion and fire include one in September at a middle school in Frisco, Tex., in which one student had to be flown to a burn center for treatment. In a 2004 case in a Seattle high school, the science teacher was severely burned by four-foot flames during the rainbow demonstration, though she was repeating an experiment she had done many times before without incident. As in the 2006 Ohio case in which Ms. Weber and other students were burned, the explosion occurred when the teacher was adding more methanol to one of the dishes.

Ms. Poole, who was said to be distraught, did not respond to a message left at her building Friday. Devon Puglia, a spokesman for the city's Education Department, said officials were still investigating the accident.

"What the students experienced yesterday was an extremely unfortunate accident," he said in an email, adding, "We're determined to get to the bottom of this so that it never happens again."

Methanol is highly flammable and has a very low boiling point, chemistry experts said, so that it quickly produces flammable vapors even at room temperature. Any spark, even static electricity in winter, can ignite them, said David J. Leggett, 68, a veteran consultant on lab safety issues in Playa del Rey, Calif.

"I would frankly question the use of methanol in an experiment that is simply going to say look kids, look at the colors," he said.