JOPLIN, Mo. — A 13-year-old student wearing a mask and a long, black trenchcoat fired an assault rifle into the ceiling at his school yesterday morning after confronting a pair of students and administrators, telling them “please don’t make me do this,” officials said.

No one was injured, and the boy, who police said was following a well-thought out plan, was taken into custody.

The seventh-grader pointed the gun at the two students, Principal Steve Gilbreth and Assistant Superintendent Steve Doerr, telling them “not to make me do this,” Superintendent Jim Simpson said.

He then fired the shot into the ceiling, breaking a water pipe, and said again “Please don’t make me do this,” Mr. Simpson said.

Messrs. Doerr and Gilbreth persuaded the youth to leave the building, and he was confronted by two police officers who had their weapons drawn.The student dropped the rifle and was taken into custody, Mr. Simpson said.

“It was a very close call,” he said.

Police Officer Curt Farmer said officers found a note in the student’s backpack indicating that he had placed an explosive in the school, which has about 750 students. Students were moved to nearby Joplin Memorial Hall to be picked up by their parents.

The backpack also contained military manuals, instructions on assembling an improvised explosive device, and detailed drawings of the school.

“This was quite well thought out,” Mr. Farmer said. “He had been planning this for a long time.”

Mr. Farmer said police believe they know where the student got the weapon but would not disclose those details. He said it was not uncommon for people in the area to own high-power firearms.

The student, whose name was not released, was wearing a T-shirt over his head with holes cut out for eyes, Mr. Farmer said.

The school was closed while police and Missouri State Highway Patrol officers searched the building with bomb-sniffing dogs.

Mr. Simpson said authorities did not know whether others were involved, and school officials said they could not discuss a motive and would not have access to the student’s records until the search was completed.

Police searched a home in the area and officers were seen leaving the house carrying two rifles, the Joplin Globe reported. Police roped off a half-block area around the house and would not disclose specifics about the search.

The shooting happened about 7:45 a.m., 10 minutes before the start of classes.

“A lot of the kids were scared,” an eighth-grader, Deron Moore, said. “After they said on the intercom that there was someone with a gun, I kind of went into shock.”

A woman who was dropping her son off at the school said she saw Mr. Gilbreth “waving crazily” and police cars pulling up behind her.

“Police were heading into the school with weapons drawn, and the principal was pointing to the east side of the school,” Blake Spivak told the Joplin Globe.

Former advertising director for the newspaper, Ms. Spivak said she and her son, Cooper, stayed in the car while Mr. Gilbreth walked back into the school flanked by armed officers in protective jackets. She said an officer told her to get down or get her car away from the scene.

Joplin, which has about 40,900 residents, is in southwest Missouri, about 140 miles south of Kansas City. It is about 10 miles from the small town of Riverton, Kan., where five high school students were arrested in April and accused of plotting a school rampage there.

Schools across the country have been on alert since three deadly school shootings in three states in the span of a week, and several schools have been locked down or closed entirely during the past two weeks because of threats.

In Pennsylvania Amish country Monday morning, church bells tolled across the region in remembrance of the five young girls who were shot to death at their one-room schoolhouse one week earlier.