The argument between the two students during first lunch began innocently enough — with some trash talk about knowledge of football statistics. By the time it was over, a teacher had to be hospitalized with a brain injury, an assistant principal had a grapefruit-sized bruise on his neck and a 16-year-old student at St. Paul Central High School faces felony assault charges now for allegedly attacking the school staff members who tried to break up a fight.

It is the type of student-on-staff school violence that Ramsey County Attorney John Choi said has been increasing at an alarming rate.

"We all need to start paying attention to what is happening in our schools and question how we are raising our children as a community," he said Tuesday.

The incident also drew a strong response from the teachers union, which said Tuesday night that it is filing a petition for mediation, a first step toward a possible strike move over the issue of school safety. The union and district have been in negotiations since May.

St. Paul Schools Superintendent Valeria Silva said in a letter sent to teachers Wednesday that a student was expelled from the district Tuesday. It would be the first time since Nov. 2009 that the school board has voted to expel a student, a district spokeswoman confirmed.

District officials did not confirm if the student expelled was the one who brought a loaded gun to St. Paul Harding High School in October. The expulsion was the first to occur under Silva's watch.

According to witnesses, police reports and court documents, a teacher was choked into unconsciousness after trying to break up a fight Friday between students in the Central cafeteria. The students — one a senior and the other a freshman — had been arguing over football statistics. The talk escalated into a fight, quickly broken up by staff members, said Central student Ayantu Bikila, who was sitting nearby.

"It was over in like 10 seconds," she said. "The two boys were already pulled apart."

But then the older brother of the freshman became incensed at the senior for fighting his brother, she said.

That is when the 55-year-old teacher intervened, attempting to pull the older brother away from the boy who had fought the younger brother. "He [the teacher] was holding him away, saying, 'Just go away, just go away,' " Bikila said. "But he wouldn't."

According to court documents filed Tuesday, the 16-year-old — who allegedly had the teacher in a chokehold — picked up the teacher and slammed him into a table and chair, before slamming him to the floor. The teacher passed out for 10 to 20 seconds.

The suspect also is accused of pushing an assistant principal into a wall, leaving that man with a "grapefruit size bruise on his neck." The younger brother also is accused of assaulting staff members and the assistant principal, allegedly punching "him repeatedly in the upper chest."

When advised he was under arrest, the student allegedly laughed. Later, in an interview with police, he denied that he assaulted any staff members. He has been charged with felony third-degree assault, plus two gross misdemeanors of fourth-degree assault and obstruction of legal process. He pleaded not guilty to the charges in juvenile court Tuesday and is scheduled to go on trial Jan. 4.

The teacher was admitted to a hospital Friday after reporting tingling in his hands and pain in his back and neck, the charges say.

In a statement Tuesday, Choi said he was "deeply troubled by this case and what it represents: an alarming increase in violence perpetrated by students against school officials throughout Ramsey County.

"We cannot and will not tolerate this type of behavior in any of our schools."

Choi said that the juvenile case was the 27th to be presented to his office this year under a gross misdemeanor statute that protects school officials from being assaulted or harmed. He said that the cases have almost doubled in the past year, and also represent a 60 percent increase over the previous five-year average.

Contacted at her office earlier Tuesday, Central Principal Mary Mackbee said she could not comment on the incident and deferred questions to district administrators. But Mackbee, who has led Central since 1993, acknowledged it was "one of the worst" cases she has faced.

Police incident data for the St. Paul schools show that police calls to Central actually lag behind some of the city's other, smaller high schools. Of the eight times police were called to the school between Sept. 18 and the end of October, one incident involved a reported assault and another involved a weapon. In early November, police and staff recovered two weapons at or near the school — an unloaded gun and a knife.

Bikila said fights are not common "and nobody has ever touched a teacher or anything."

At a news conference Tuesday, Choi and Silva talked about steps they plan moving forward.

Choi said the issue of violence against teachers is not St. Paul's alone — the district's schools have accounted for about half of the fourth-degree assault charges presented to his office.

So he is developing a community task force to look at youth violence in the schools and the broader community and said that Silva has agreed to join. Choi said he will reach out to other superintendents to participate.

The task force is likely to start work early next year.

Late Tuesday, the St. Paul Federation of Teachers said the assault led it to file a petition for state mediation.

"This step is required by state law to trigger our teachers' right to strike," union president Denise Rodriguez said in a written statement. "Teachers don't want to walk away from their classrooms or their students, but if our school climates are not safe and equitable environments for learning, that is a step our members may need to take."

Reached by phone, she said, "This was the breaking point. Teachers are asking, 'What's the union's reaction to this? What are we going to do to make sure our students, teachers and support staff feel safe in school?' This will force the district to come to the negotiations table."

Silva said the teacher who was attacked is beloved and was doing what all educators would do to protect kids.

As a 28-year employee, she said she thought she'd seen everything: "Every day, unfortunately, we are all surprised, and unfortunately, very sadly surprised."

Staff writer Mary Lynn Smith contributed to this report. 651-925-5036 651-925-5041