A 15-year-old girl plunged a knife into the back of a classmate in front of dozens of students eating lunch in the cafeteria of Kramer Junior High School in Anacostia yesterday, witnesses and police said.

Several students, describing the sudden attack, said the girl pulled out a kitchen knife with a long blade, walked toward a student who had just teased her and stabbed him once in the middle of the back.

In the ensuing panic, as students and an assistant principal rushed to the aid of the collapsed boy, Joseph Harris, 13, the girl calmly returned to her seat with the knife in her hand, two witnesses said.

"She walked back to the table, took a napkin, wiped the knife and laid it on the table," said Sakina Plummer, 15. "Then she walked out of the lunchroom."

The assault in the lunchroom was the third stabbing at the school in a year, and by far the most serious. The knife, with a blade longer than 8 inches, went through Joseph's body, the first police officers on the scene reported.

At one point, the assistant principal pulled up the victim's shirt to reveal a wound that, in the words of Elliott Baker, 14, was "gushing and bubbling" blood. He was rushed to Children's Hospital, where he underwent surgery and was listed in critical condition. His condition was upgraded to serious late last night, according to a hospital spokeswoman.

Almost two hours after the stabbing, which occurred about 12:50 p.m., 7th District police officers arrested the 15-year-old girl in the 1500 block of Butler Street SE and charged her with assault with intent to kill.

Police, who did not release much information because the incident is still being investigated, said the stabbing stemmed from "an apparent verbal altercation" between the two students. In interviews with about a dozen students, some of them witnesses, the following account emerged:

All the ninth-graders and about half the eighth-graders were in the lunchroom, which is in the basement of the school at 17th and Q streets SE, when the boy began teasing the 15-year-old.

The two students dislike each other, several classmates said, although it is not clear why they do not get along. The boy, over the course of a few minutes, approached the girl several times, shoved her chair at least once and, in general, was "messing" with her, said Sakina and another classmate, Phoebe Osborne, 15.

After one such encounter, Joseph turned to return to his table. It was then, the students said, that the girl suddenly produced the knife and attacked him from behind. At first, several students said they did not realize what had happened because the knife was quickly withdrawn and the boy seemed to just stumble.

"He looked at me and he said, 'I got stabbed, I got stabbed,' and then he fell down," said Elliott Baker, an eighth-grader.

"I saw him running back and forth and he was saying 'Ahhhh,' " said Montrella Cowar, 13, an eighth-grader. "Then I saw the blood in his back."

Joseph said he was unable to breathe, and an assistant principal, whose name could not be learned, performed cardiopulmonary resuscitation. The first police officers on the scene quickly learned the name of the 15-year-old girl, who walked out of the school during the confusion.

There have been five stabbings inside District schools since last February, three of them at Kramer. The last one at Kramer occurred Jan. 24, when a 14-year-old received a minor stab wound under his arm. In February 1990, another 14-year-old student there was stabbed and slightly injured. The most serious recent incident occurred at Eastern High School in December, when an 18-year-old student was stabbed in the stomach during an argument.

Periodic school violence has almost always occurred on street corners near school grounds, and school officials say incidents like the one at Kramer are unusual. Security was increased this year in part to curb violence on school property, and there is currently a guard and an aide in all secondary schools, including Kramer.

R. David Hall, president of the D.C. Board of Education, said that the Kramer Junior High School community has complained before about security and other problems at the school. He plans to discuss these concerns next week, and he said the stabbing makes the visit to Kramer more urgent.

Hall said the presence of a large knife bolsters his argument for placing metal detectors in all D.C. schools. "That is the only way to keep these weapons out," he said.

Staff writers Keith Harriston and Nathan McCall contributed to this report.

Feb. 5, 1990: Kenka R. Broadnox, 13, was critically wounded when she was stabbed in the chest outside Lincoln Junior High School. Another 13-year-old girl was charged in the incident.

Feb. 6: Charmisa Glover, 14, was stabbed at Kramer Junior High School. A 16-year-old classmate was charged.

April 5: James Offut, 16, was stabbed to death on the playground of Shaw Junior High School during a fight with several other youths.

Dec. 19: Gerald Mooney, 18, was critically wounded in a hallway of Eastern High School during an argument over a bag of corn chips.

Jan. 23, 1991: Jerome Vines, 14, was stabbed during a dispute with another student at Kramer Junior High School.

Feb. 13: Joseph Harris, 13, was stabbed in the back during lunch at Kramer Junior High School.