ST. PETERSBURG — A fight at a school bus stop escalated Thursday morning, resulting in a 16-year-old girl being stabbed in the back by a fellow Gibbs High student.

St. Petersburg police say the girls — ages 15 and 16 — have been involved in an ongoing disagreement and began a "physical fight" at the stop near Paris Avenue and Sixth Street S.

As their bus approached, the 15-year-old "disengaged from the fight, where she then went and grabbed the steak knife that was placed in a nearby sewer run-off," according to an arrest affidavit.

After stabbing the victim the back, the 15-year-old threatened the victim's mother with a pair of scissors, before running onto the bus, police said.

"(She) was holding the scissors above her head while approaching the victim," the arrest affidavit said.

The girl was taken into custody and faces three felony charges: aggravated battery, aggravated assault and possession of a weapon on school property, police said. The bus stop is considered school property, they said.

The victim was listed in stable condition Thursday at Bayfront Health St. Petersburg.

In a phone call to parents of the other 25 to 30 students on the bus, Gibbs principal Reuben Hepburn recounted what happened. He explained that law enforcement was investigating and might contact some of those who may have witnessed the attack on their way to school.

"This is a sad and disappointing situation for me as principal," Hepburn said, adding that parents can contact him during regular school hours with questions and concerns. "This incident does not represent the excellent student body we have and the caring environment we strive for at Gibbs High School."

Pinellas County's student code of conduct, which is provided to students at the start of each school year, states that "cutting objects of any kind" are prohibited on school buses.

"We have the same safety protocols on buses that we have in schools," said school district spokesperson Lisa Wolf. "Just like they are expected to follow the code of conduct in schools, they are expected to follow the code of conduct on the bus."

The district will wait until police are done investigating to discipline the 15-year-old student, she said, adding that student services representatives are available to students who may have witnessed the attack and need support.

On Thursday afternoon, police were still reviewing video of the attack, though the agency did not confirm whether the footage was recorded by a bystander, the bus surveillance system or both. Detectives are continuing to interview witnesses, police said.

"Detectives are trying to talk to as many kids of possible to string together a timeline," said police spokesperson Samantha Williams.

Contact Megan Reeves at mreeves@tampabay.com. Follow @mareevs.