Saying she was "deeply concerned" about teachers threatening to strike over school safety, St. Paul schools Superintendent Valeria Silva said Wednesday she was committed to finding solutions but cautioned the district can't afford all of what the union wants.

Silva told reporters she was "very surprised" to hear the St. Paul Federation of Teachers warning the spike in student violence could lead to a strike.

The district and the union are in talks now over a new two-year contract. Part of that involves language on how to approach school safety. Union officials said they've proposed more school staff, training and smaller class sizes as the way to improve safety.

Silva said the district sent a counterproposal to the union including safety training and staff at each school but hasn't heard back.

Collectively, she said, the district can't afford all the union's contract demands, which she said also include increases in teacher pay scales amounting to 9 percent over two years.

Until today, it seemed negotiations were moving "in the right direction," the superintendent added.

Central High School Principal Mary Mackbee talked about school safety issues at the St. Paul Public Schools administration building Wednesday, Oct. 21, 2015. Behind her, from left, is Superintendent Valeria Silva, Harding High School Principal Doug Revsbeck and Como Park Senior High School Principal Theresa Neal. Riham Feshir | MPR News file

The teachers union move comes after Ramsey County Attorney John Choi charged a 16-year-old Central High School student with a felony in connection with a brutal assault on a teacher last Friday, the latest in a spate of similar cases of violence in schools across the county this year.

"These incidents involving students assaulting teachers across the county have been increasing at an alarming rate," Choi said Tuesday.

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Friday's incident marked the 27th such case in Ramsey County during the 2015 calendar year — a 60 percent increase over the county's previous five-year average, Choi said.

Choi and Silva announced plans Tuesday to form a task force to address the surge in school violence.

St. Paul Federation of Teachers: Statement on school climate

Hours after that, however, the union sought mediation and threatened a strike.

"If our school climates are not safe and equitable environments for learning, that is a step our members may need to take," union president Denise Rodriguez wrote. "We can wait no longer."

Silva and the union both favor a school safety and discipline plan known as restorative justice. The program tries to bring students together to settle conflicts and repair the harm done.

"I am deeply committed to finding common ground between the district" and the teachers union, Silva said. "There's nothing more important to me at this point."