Just days after their school year started, 400,000 Chicago students will be out of school indefinitely as their teachers go on strike Monday in the country’s third-largest school district.

Almost 30,000 teachers and support staff are off the job after their union and school officials failed to reach a contract agreement despite 10 months of negotiations. The walkout is Chicago’s first teacher strike in 25 years.

The flood of out-of-class students from almost 700 schools is prompting police to beef up operations. Police Superintendent Garry McCarthy said the department will be “emptying out our offices” to send officers in administrative jobs out on the street.

“The kids in Chicago belong in the classroom,” Chicago Mayor Rahm Emanuel told reporters Sunday night, after the talks failed. “Our kids do not deserve this.”

Sticking points for the union and the school system included issues such as compensation, job security, merit pay and an evaluation system.

“Negotiations have been intense but productive, but we have failed to reach an agreement that would prevent a labor strike,” Chicago Teachers Union President Karen Lewis told reporters late Sunday night.

The district said 144 schools will be open with limited hours, but parents are strongly encouraged “to first explore other options for their children.”

“We know that a strike will put a strain on many families, and no one will be hurt more by a strike than our students,” Chicago Public Schools said on its website.

In addition to figuring out where to send their children, some dismayed parents are also concerned about what students might get into.

“If the kids are not in school, they’re out getting into some kind of trouble … when they should be in school, learning,” Shatara Scaggs said.

Dozens of civic and faith-based facilities are opening their doors to provide safe outlets to students during the strike.

Young Chicago Authors, for example, will provide programs for students in grades 6 through 12 this week.

“In collaboration with core performance artists and special guests, young people will see the power of their voices in action through film, performance and discussion,” the group said.

Minutes before the union announced the impending strike, the president of Chicago’s school board said officials offered the city’s teachers a contract including pay increases and other measures they’d requested.

“We’ve been as responsive as we know how,” David Vitale told reporters.

Vitale said the package offered by school officials effectively guaranteed pay increases for four years, does not include merit pay and offers “some give on the evaluation system.”

“The average teacher will get a 16% raise over that (4-year) period” at a time when the city’s fiscal situation is on edge, Vitale said of the offered deal.

Lewis, the union president, acknowledged that “talks have been productive in many areas.”

“We have successfully won concessions for nursing mothers and put more than 500 of our members back to work. We have restored some of the art, music, world language, technology and physical education classes to many of our students. The board also agreed that we will now have textbooks on the first day of school, rather than have our students and teachers wait for up to six weeks before receiving instructional materials.”

But issues such as health benefits and evaluation procedures remain.

Lewis said she was concerned about the possibility of a large termination of teachers under a new evaluation system.

“We are also concerned that too much of the evaluations will be based on students’ standardized test scores. This is no way to measure teacher effectiveness at all,” she said.

The school year came to an abrupt halt not long after it began.

Some students in the district began class on August 13. Others, on a different schedule, started on September 4.

“In the morning, no (union) members will be inside our schools,” Lewis said, adding that teachers will march on picket lines and talk to community members.

Mayor Emanuel said he was disappointed at the impasse Sunday night, saying he believed the offers laid out were fair to both teachers and taxpayers.

But, he said, his staff is ready to keep working to “close the gap” between negotiators.

— CNN’s Greg Botelho, Ted Rowlands and Greg Morrison contributed to this report.