A 16-year-old student at a Northeast Washington high school has been arrested for allegedly dropping a fire extinguisher from a second-floor gymnasium balcony onto his physical education teacher, authorities said yesterday.

D.C. police and school officials are investigating the incident, which took place at 3:10 p.m. Tuesday at McKinley Technology Senior High School, in the 100 block of T Street NE.

Police arrested the youth at the school Tuesday on suspicion of aggravated assault while armed, said Capt. Keith Williams of the School Safety Division. Police believe an argument started when the teenager did not dress for gym class and the instructor told him he could not participate, Williams said. The teacher then sent the youth out of class, and he went to the second floor and waited to drop the extinguisher on the instructor as he walked by, according to police.

"The consequences could have been so severe," Williams said. "Whatever they were arguing about, it doesn't warrant dropping a fire extinguisher on his head."

The teacher, who was clipped on the back of his head, was taken to Howard University Hospital, said Daniel Gohl, principal of McKinley Tech. The teacher suffered a concussion. He was released from the hospital yesterday and was resting at home, said Gohl, who, along with parents familiar with the incident, declined to give the teacher's name.

D.C. police officers who are assigned to the school found the teacher drifting in and out of consciousness, Williams said, but were able to gather information that led them to the teenager.

The sophomore, whose name is being withheld by authorities because of his age, could be detained for as long as five days without being charged, according to the D.C. attorney general's office. He remained in custody yesterday while authorities sorted out what happened.

School officials are initiating unspecified disciplinary action against the student, Gohl said. "I take this to be a very serious incident. It is being treated with that degree of severity," he said.

When the teacher was struck, some students ran to his aid, while others sought help.

"This incident is not a reflection on McKinley. It was one child who went through whatever he went through," said Cherita Whiting, president of the McKinley Tech PTA. She added that she and her 15-year-old son, who attends the school, did not know the student. "The good at the school outweighs the bad."

The teacher, who has been at the school for two years, is well liked, Gohl and Whiting said. Gohl said students have signed a get-well card for him.

Whiting said she packed her son and four other students into her sedan this week to visit the teacher in the hospital.

"He actually smiled, knowing there were students downstairs who wanted to see him," she said. "He mentioned that's why he's going to get better -- for the kids."

The teacher has a wife and a newborn baby, she said. The PTA is planning to purchase a gift for the baby to demonstrate the group's support for the teacher, Whiting said.

Gohl sent a letter to parents notifying them of the incident, the second apparent act of violence at a D.C. school this week. Also on Tuesday, a 16-year-old student at Coolidge Senior High was arrested on assault charges after fighting with the principal in the hallway of the school, in the 6300 block of Fifth Street NW, authorities said.

Staff writer Henri E. Cauvin contributed to this report.