Chicago Mayor Rahm Emanuel has announced that he will seek a court order to end the first teachers’ strike in the city in 25 years, which escalated on Sunday when the teachers’ union decided to extend their walk-out.The strike has cancelled classes for 350,000 kindergarten, elementary and high school students in the United States’ third-largest school district and will enter its sixth day on Monday.It risks friction within President Barack Obama’s political coalition, where many Democrats differ over approaches to education reform, ahead of the November 6 Presidential election against Republican Mitt Romney. Emanuel is Obama’s former top White House aide.The mayor called the strike "illegal” on Sunday and said he would go to court to seek an injunction to block it."I will not stand by while the children of Chicago are played as pawns in an internal dispute within a union,” Emanuel said, adding that the union walked out over issues that are not subject to a strike under Illinois state law.Education reformsTeachers revolted last week against sweeping education reforms sought by Emanuel, especially evaluating teachers based on the standardised test scores of their students.They also fear a wave of neighbourhood school closings that could result in mass teacher layoffs. They want a guarantee that laid-off teachers will be recalled for other jobs in the district.The showdown left in doubt a deal on wages, benefits and education reforms for 29,000 unionised teachers that negotiators had hoped would end the biggest labour dispute in the US in a year.Union delegates will reconvene on Tuesday to discuss the feedback from members, union President Karen Lewis said, adding that parents should plan for their children to be out of school until at least Wednesday.Before the meeting of delegates on Sunday, Lewis had called the agreement a "good contract”. But after the decision to extend the strike she backtracked, saying: "This is not a good deal. This is the deal we got.”