St. Paul schools Superintendent Valeria Silva issued a call Tuesday for district and union negotiators to meet weekends, nights, whatever it takes, she said, to hammer out a new teachers contract — and prevent a strike.

The St. Paul Federation of Teachers, for its part, is ready to go, "absolutely," union secretary Nick Faber said.

The move, however, will require persuading the state Bureau of Mediation of Services to schedule additional mediation sessions between now and Feb. 24 — when the union's executive board has scheduled a strike authorization vote.

Barring the scheduling of the sessions, the two sides — at odds for nearly nine months over a new two-year contract — have just one planned meeting, on Feb. 20. They also still have considerable differences to resolve in the state's second-largest district, with nearly 38,000 students and about 3,200 teachers. Union leaders on Monday said they plan to have members vote Feb. 24 on whether to give them permission to call a strike.

On Tuesday, Silva joined school board Chairwoman Mary Doran in announcing intentions to intensify bargaining at a news conference during which they also outlined how a strike could affect families and students. It would result not only in school closings, they said, but also the suspension of athletic activities, preschool and after-school programs.

In addition, they noted, students might face an extended school year, forcing delayed graduation ceremonies for high school seniors once classes resume. Silva also raised the specter of many students missing out on breakfast and lunch.

The laying out of the details, including the potential layoffs of nonteaching staff, disappointed union leaders.

"It seems that they're focusing more on the strike part than on the talks," Faber said.

The two sides remain divided on union proposals to limit class sizes and expand preschool opportunities — priorities that union President Mary Cathryn Ricker said Tuesday would take the contract into the 21st century.

Silva said the proposals were unaffordable — potentially costing the district as much as $150 million altogether.

Two years ago, the union made inroads on the class-size issue by negotiating with the district a special memorandum of agreement stating that if the district could afford it, class sizes would be kept in the "lower range" of numbers approved as part of the district's Strong Schools Strong Communities strategic plan. This round, the federation has pushed to cap the class sizes, and in the face of district objections, now proposes a phase-in of the plan, Ricker said.

Matt Mohs, the district's chief academic officer, said Tuesday that a phase-in "makes it seem more affordable in the short term, but the [ends] are essentially the same. We have to look at what the full implementation would be."

Still, he added, "we are hopeful that we can find some common ground around class sizes."

Also still to be resolved are wage-and-benefit issues that under the most recent proposals could cost the district about $16.5 million or more over the next two years. The $16.5 million represents the district's offer, which calls in part for an honoring of built-in seniority raises as well as a cost-of-living increase, Mohs said.

Currently, the average St. Paul teacher is paid about $68,000 annually.

The union's initial wages-and-benefits proposal would have cost taxpayers an additional $23.7 million over two years, by the district's estimate. The federation since has made a new offer, but the district did not have a cost estimate for that package Tuesday.

But, Mohs said, "the main tensions here [in negotiations] are not about wages and benefits."

St. Paul teachers last voted to strike in 1989, but the walkout was averted at the last minute. The most recent teacher strike was in the Crosby-Ironton district in 2005, which lasted eight weeks, the second-longest strike in state history.

Last last month, 157 of the 334 districts tracked by Education Minnesota had reached contract agreements.