September 10. That’s the date the Chicago Teachers Union says it will walk out on strike unless teachers have a contract agreement.

The union’s House of Delegates voted unanimously Thursday afternoon to trade their chalk for picket signs a week from Monday if no deal is reached. That means many CPS students would have only four days of school before a walk out.

Chants of “strike” and “CTU” emanated from the South Side union hall where teachers met.

Union president Karen Lewis said negotiators have been to more than 45 bargaining sessions since November.

“This is the step that you have to take, and we are taking all of the steps that we have to take,” said Lewis. She indicated that negotiations advance every time the union inches closer to a walkout.

Delegates left the union hall with armfuls of picket signs.

The next negotiating session is Friday.

Lewis said she was hopeful that a contract agreement could be reached before the strike date. She said the union’s priorities at the bargaining table are wages and job security. It also wants a “better school day,” including class size caps and increases in support staff such as social workers, counselors and nurses.

Chicago Public Schools released details Thursday of a strike contingency plan that would keep 145 schools open half days with central office staff and non-union employees leading kids in such activities as journaling, art, and computer-based games. The schools would serve meals.

The union filed notice late Wednesday of its intent to strike.

One union delegate, a high school math teacher and CPS parent who asked not to be named, said he considers the strike contingency plan, which could cost up to $25 million, “just another direct slap in the face” to teachers.  “Hell no, we’re not sending our kids there,” he said.

In a written statement, schools chief Jean-Claude Brizard said “if our priority is our kids, then strike should never be an option.”  About 350,000 students would be affected by a teacher walkout. The district says varsity sports would be canceled.

“We need to take advantage of each of the next 11 days and work until we reach a fair resolution for our teachers that will allow our kids to stay in school where they belong,” Brizard said in the statement.

The union’s House of Delegates has the power to cancel or revise the strike date.

The last strike in the school district was in 1987.

Becky Vevea contributed reporting.