That was the only warning students on the yellow school bus had yesterday morning in West Philadelphia before the roar of gunfire obliterated the morning monotony of their commute to school.

The 15-year-old who barked the threat blasted a 17-year-old classmate in the head, police said, as nine other students on the bus, idling curbside outside the Andrew Hamilton School at 57th and Pine streets, watched in helpless horror.

The older student remained in critical but stable condition at Children's Hospital of Philadelphia yesterday, with doctors scrambling to discern the damage caused by a bullet that entered his head near his ear, said Capt. Ben Naish, commander of Southwest Detectives.

Officers quickly caught alleged shooter Taryale Petter, of Reinhard Street near 61st, in West Philadelphia, a few blocks away at 61st and Cedar streets. He faces adult charges of aggravated assault and related offenses.

Both students, whose names weren't released because they're juveniles, are enrolled at Community Education Partners Miller School, a disciplinary school at 43rd and Westminster, school district spokesman Fernando Gallard said. The bus had stopped at Hamilton to pick up more students.

"I'm still in shock," said Kia Woodbridge, who rushed to the 18th Police District headquarters at 55th and Pine to get her son Nijul Porter, who had been sitting near the victim and witnessed the shooting. "I just got him transferred. It was his first day on that bus. Thank Jesus my son made it off the bus."

Naish said the teens had a previous conflict that led to the shooting. Investigators know who owns the shooter's gun, a .25-caliber semiautomatic, and are trying to determine how the 15-year-old got it, Naish added.

Chaos erupted after the 8:10 a.m. incident, with kids diving for cover as the gunman loped to the front of the bus, Porter, 17, said.

There, the shooter pointed his gun at bus driver Jean Kernizan, ordering him to open the folding doors so he could flee.

"The gun was against my head. So I opened the door," Kernizan said later to reporters outside the 18th district, two blocks from the shooting scene.

The school district requires newer buses to have video cameras inside to monitor misbehavior, and this bus had one, Gallard confirmed. It's unclear whether the camera captured the shooting.

The district typically doesn't transport high-schoolers on yellow buses, but disciplinary students are an exception, Gallard said. To improve safety, the district requires a school staffer to serve as a bus attendant on buses transporting disciplinary students, Gallard said.

This bus had an attendant, Gallard said, but the shooter gave little warning of the violence he unleashed.

In a district where every high school has metal detectors, parents like Woodbridge wondered whether more precautions - like frisking kids before they board buses - might be necessary.

"They want me to put him back on the bus? Then put some metal detectors on the bus," Woodbridge fumed.

But Gallard urged parents and community members to address the "real problem" yesterday's shooting presented.

"This is not about school bus safety. This is about handgun safety," Gallard said.