Updated 09/10/12 – 5:12 p.m.

CHICAGO (CBS) — Chicago Public Schools teachers went on strike for the fist time in a quarter century on Monday, after the latest contract talks broke down Sunday with no deal to avert a walkout.

Chicago Teachers Union President Karen Lewis said late Sunday there had been some progress in contract talks, but “we have failed to reach an agreement that will prevent a labor strike.”

The impasse left the city with the first teachers’ strike since 1987. The union had set a midnight deadline for a walkout.

CPS and CTU negotiators were back at the table Monday, working through what the mayor said are two remaining issues: a new teacher evaluation system, and hiring discretion for CPS principals. As of Monday evening, there had not been any word on the progress of the latest round of contract talks, which were held at the offices of a Chicago law firm.

The union conceded wages are not the hold-up any longer, but defended going on strike, even though the two sides are close to an agreement on salaries.

“Layoff and recall are not issues in which we’re legally entitled to strike over, but I would say, obviously those things are important for a contract,” CTU Vice President Jesse Sharkey said.

RELATED: THE KEY ISSUES

Teachers began to picket at schools shortly before 6:30 a.m., and students and parents were left looking for alternatives. The Chicago Public School systems is opening about 140 schools for children from 8:30 a.m. to 12:30 p.m.. However, no instruction will take place. In addition, churches, libraries and community organizations will be providing students with activities.

That is a worry for many parents.

“I have some concerns about who is going to be taking care of my kids, if the child to adult ratios will be too high,” Tiffany Williams told CBS 2’s Vince Gerasole.

LISTEN: WBBM Newsradio’s John Waelti Reports

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A group of pickets were in front of Chicago Public Schools headquarters early Monday, chanting and carrying signs.

RELATED: List Of “Children First” Locations

“We are committed to staying at the table until a contract is in place. However, in the morning, no CTU members will be inside our schools. We will walk the picket lines, we will talk to parents, we will talk to clergy, we will demand a fair contract today, we demand a fair contract now,” Lewis added. “Until there’s one in place, that our members will accept, we will be on the line.”

Late Sunday night, Mayor Rahm Emanuel said he was “disappointed” in the union’s decision to continue with a strike.

“I am disappointed that we have come to this point, given that even all the other parties acknowledge how close we are, because this is a strike of choice,” Mayor Rahm Emanuel said late Sunday. “Because of how close we are, it is a strike that is unnecessary.”

The mayor said CPS negotiators are available at any time to resume contract talks, and have asked the union to postpone a strike. He insisted the two sides have reached an agreement on financial issues, leaving only teacher evaluations and teacher retention issues to negotiate.

“All that makes it a strike by choice, because they are not financial issues,” Emanuel said. “This is totally unnecessary, it’s avoidable, and our kids do not deserve this.”

He also shrugged off any criticism that he has not personally attended contract talks.

“It’s not about my presence, it’s about reaching agreement,” he said. “As I believe, this is an honorable deal. It’s an honest compromise between both parties, without anybody having to compromise principle.”

Vitale said he believes Chicago Public Schools officials have made their best possible offer to teachers.

“There’s only so much money in the system. There’s only so many things that we can do that are available to us,” Vitale said. “At this juncture, it is clearly their decision. … We’ve done everything we can.”

Vitale said the district’s latest offer included a 3 percent raise for teachers in the first year, then 2 percent raises in each of the next three years of the proposed deal.

READ: CPS Handout On Contract Proposals

Vitale said the district’s offer would also allow teachers who lose their current positions from school consolidations to follow their students to the consolidated school, “to the extent that positions are available.”

In the case of teachers whose positions are eliminated to a school closing, they could either choose to take three months of severance, or take five months to apply for other CPS teaching positions, and be given preference for interviews with CPS principals.

For teachers that lose their positions for other reasons, those teachers would have recall rights for one year, for the same unit and position.

Lewis said the two sides were close to agreement on a contract, but not close enough.

“We are not far apart on compensation, however we are apart on benefits,” Lewis said. “We want to maintain the existing health benefits.”

Lewis said the union is also concerned that a proposed new teacher evaluation system “could result in almost 6,000 teachers – or nearly 30 percent of our membership – being discharged within one or two years. This is unacceptable and leads to instability for our students.”

She said the new evaluation system would rely too much on students’ standardized test scores.

“This is no way to measure teacher effectiveness at all,” she said.

CPS had already put in place a contingency plan for the strike. The district was prepared to open 144 school sites for various activities, and so that children can eat lunch and breakfast in a district where most of the 402,000 students qualify for free meals, because they are from low-income families. The sites will be open from 8:30 a.m. to 12:30 p.m. each day of a strike.

RELATED: List Of “Children First” Locations

The union has been prepping its members all weekend to get ready for the picket lines. The union’s strike headquarters on the Near West Side was a hub of activity on Saturday and Sunday.

Union members handed out picket signs, stickers reading “Yes To Education Justice,” and red CTU T-shirts to hundreds of teachers Sunday morning at Teamster City union hall at 1642 W. Van Buren St.

By 5:30 p.m. Sunday, they had run out of T-shirts and closed up shop for the night, although there were still plenty of signs to pass out to teachers and their supporters if and when they hit the picket lines Monday morning. If it does, teachers have been asked to show up at their schools at 6:30 a.m. Monday to picket.

The union also planned a 3:30 p.m. rally on Monday outside Chicago Public Schools headquarters, at 125 S. Clark St.

Principals and administrators have been prepping their schools and stocking up on supplies in case of a strike. Non-union CPS staffers will be supervising at the so-called “Children First” sites.

Dr. Tatia Beckwith, principal of William H. Ray Elementary School, said, in the event of a strike, each school open for activities is not to have more than 25 children per adult.

“We’ll have activity leaders that will take them to different rooms in the school, for different activities,” she said. “We will have art supplies. We have plenty of books for reading. We have some videos. We have centers set up in the rooms, for different kinds of play – a lot of board games, cards, those types of things.”

Police Supt. Garry McCarthy said the Police Department has been planning for weeks for a possible strike. He said the department’s plans include three areas: providing security at school sites and other locations open for activities for students during the strike; providing security at any teacher protests; and deploying extra officers in uniform to ensure safety for children who are not in school.

“We’re emptying out our offices. We’re taking out officers who are on administrative duties, we’re shutting down administrative duties, we’re putting those officers on the street to deal with potential protests at various locations throughout the city,” he said.

The department is also deploying officers to all 144 public school locations where students will be able to take part in activities provided by the district during the strike.

The superintendent also said tactical officers, who typically dress in plain clothes, will be deployed in uniform and work longer hours “to ensure that we provide enough deployment throughout the city, where children may not be in school, and might be on the streets.