Parents should plan for their children to be out of school until at least Wednesday - that's what officials are saying after the Chicago Teachers Union extended its strike into a second week on Sunday. 

The decision to continue the strike came after nearly 800 union representatives could not come to an agreement over contract details. 

"There's all kinds of stuff that they're concerned about," said Karen Lewis, the president of the Chicago Teachers Union who had played a key role in negotiating the tentative deal, as she emerged from the meeting with delegates. "This is the deal we got."

The strike will cause the 350,000 students of the nation's third-largest school system to miss six school days, uncertain when they can go back to the classroom. 

Officials are furious about the teachers' choice to keep students out of the classroom for another week. 

"I will not stand by while the children of Chicago are played as pawns in an internal dispute within a union," Chicago Mayor Rahm Emanuel said in a statement. "This was a strike of choice and is now a delay of choice that is wrong for our children. Every day our kids are kept out of school is one more day we fail in our mission: to ensure that every child in every community has an education that matches their potential."

Emmanuel is expected to take the union to court, calling the strike illegal. 

"This continued action by union leadership is illegal on two grounds - it is over issues that are deemed by state law to be non-strikable, and it endangers the health and safety of our children," the mayor's statement said. "While the union works through its remaining issues, there is no reason why the children of Chicago should not be back in the classroom as they had been for weeks while negotiators worked through these same issues."

Union delegates will meet on Tuesday and nearly 26,000 union members will need to vote on whether to ratify the new contract. 

The strike is the first for a major American city in at least six years and Chicago's first in 25 years.