A Baltimore teen-ager pleaded guilty yesterday to assault in a near-fatal, mob beating of a Northern High School student last year.

Keith Rantin, 16, was one of about 15 Northern High students who attacked Willis Reese, 16, in front of the school as he got off a bus Nov. 9 last year, court documents show.

The group beat him with fists and kicked him, police said, and one attacker smashed a baseball bat over his head until the bat split in two.

A school police officer saw Rantin pull Reese's pants and underpants off, according to a statement of facts read to the court by Assistant State's Attorney Cynthia Banks, who prosecuted the case.

"I found this a very disturbing case," said Judge Thomas E. Noel, who presided over the plea agreement.

Reese, who has post-traumatic amnesia and remembers nothing from the day he was attacked, suffered permanent brain damage although he has recovered about 85 percent of his brain function, prosecutors said.

His mother, Linda Reese, declined to comment yesterday.

Under a plea deal, Rantin will be sentenced next month by Noel to no more than five years in prison.

Prosecutors dropped attempted-murder and assault charges against three others who were indicted in the case: Sean Cox, 15, of the 6100 block of Alta Ave; Dimitrius Jones, 16, of the 5500 block of Cedonia Ave.; and Steven McCullough, 16, of the 4200 block of Belmar Ave.

Investigators could not locate a key witness who could identify the three, and without that identification, the state's attorney's office did not have enough evidence to try the cases yesterday, Banks said.

Seeking more time to collect evidence against the three, Banks asked Noel for a postponement of their cases, but the judge denied the prosecutor's request.

Noel said the case had been postponed three times, and he did not believe prosecutors were close to finding the witness they needed.

"The difficulty I had was that the state couldn't offer anything concrete as to when or where they were going to find this witness," Noel said. "They've had the Department of Social Services, the Police Department and another agency looking for this witness. No one has been able to come up with anything concrete."

Noel also said Reese's recovery was "miraculous."

"Initially, no one expected him to make the recovery he's made," the judge said. "He's functioning at 85 percent normal. They never expected this."