Updated

The Chicago Teachers Union issued a 10-day strike notice Wednesday, saying teachers in the nation's third-largest school district are ready to walk off the job for the first time in 25 years.

Union President Karen Lewis said contract talks with the Chicago Board of Education, which have been under way since November, have not yet touched on some issues that teachers are most concerned about, including wages. She said job security and teacher evaluations also are issues.

The notice means the soonest teachers could strike is Sept. 10, but it doesn't mean a strike will definitely happen.

Lewis said no strike date has been set, and the union would continue negotiating until there is a contract, even if there is a strike.

"This is a difficult decision for all of us to make," Lewis said. "But this is the only way to get the board's attention and show them that we are serious about getting a fair contract which will give our students the resources they deserve."

Some of the district's 400,000 students have already started school; the rest begin Tuesday. The last Chicago teachers strike was in 1987 and lasted 19 days. The most recent contract expired in June, when 90 percent of the more than 26,000 union members voted to authorize a strike if a contract was not reached over the summer.

Lewis claims the public schools administration "seems determined to have a toxic relationship with its employees," saying teachers have been belittled and demoralized. She pointed out that a previously negotiated raise was canceled and changes have been made to the way teachers are evaluated.

Chicago Public Schools CEO Jean-Claude Brizard issued a statement saying students "cannot afford to be removed from the classroom" just as they're starting a new longer day, and said the district will meet every day with the union to try to avoid a strike.

The school district has offered teachers a four-year contract with raises of 2 percent a year, which school board spokeswoman Becky Carroll said would cost $160 million. Lewis has repeatedly said the raise offered by the board is not acceptable.

The district also wants the union to agree to a joint committee to come up with a new system to pay teachers, other than automatic raises based on seniority. The district said that doesn't mean pay raises would necessarily be based only on merit.

"It could take a lot of forms; we have not even talked about it," Carroll said.

The board last week authorized spending $25 million in the event of a strike, which Carroll said would help ensure the tens of thousands of students who rely on the schools for two meals a day are fed and have a safe place during the day.

Much of the teachers' frustration has centered on Mayor Rahm Emanuel, who rescinded a 4 percent raise last year and then tried to go around the union in his push for a longer school day by asking teachers at individual schools to waive the union contract to work more hours. The Illinois Educational Labor Relations Board subsequently blocked Emanuel's negotiations with schools.

He still was able to lengthen the school day for children to 7 hours, starting this fall, without the union's approval. The Board of Education agreed to hire almost 500 new teachers so it wouldn't have to pay current teachers more to work longer hours.

But class size, pay and job security remain top concerns for teachers, the union said.

"We will have a contract, and it will come the easy way or the hard way," Lewis said. "If our members are on the picket line, we will still be at the negotiating table trying to hammer out an equitable agreement."