A strike last week by some 29,000 teachers in Chicago pushed long-simmering tensions over deeply divisive school improvement ideas--including changes in teacher evaluation and the takeover or closure of underperforming schools--into the national spotlight. A framework for a tentative agreement emerged last Friday, and the union's house of delegates was scheduled to meet this past weekend to vet a draft and vote on whether to call off the strike. Details of the agreement were still trickling out, but it appeared likely that the Chicago district had offered to restore some elements of a hiring preference for laid-off teachers, to slow the implementation of a new teacher-evaluation system, and to allow limited appeals under that system. Students are expected to be back in school at the beginning of this week. About 350,000 students in the district, the nation's third largest, were affected by the walkout. In Chicago's case, one such complication has been the volatile relationship between two powerful city players: (1) Mayor Rahm Emanuel, the famously combative former chief of staff for President Barack Obama; and (2) Karen Lewis, the equally outspoken president of the Chicago Teachers Union (CTU). The two have squabbled for months over Mr. Emanuel's desire to lengthen the school day, which was until recently among the shortest in urban school districts. The strike also raised delicate political questions for the White House during the tense run-up to Election Day. As in 2008, Mr. Obama is counting on the support of teachers, but his own education agenda has pushed for many of the reform ideas contested at the bargaining table. Such divisions were on display last week, as educators, clothed in CTU red, picketed in front of their schools after the walkout began on Sept. 10. Many motorists honked in support as they drove by. In the afternoon, thousands of the teachers flooded the city's downtown Loop area to attend rallies. Picketers stationed a giant, inflatable rat outside the school district's headquarters. They held up signs protesting large class sizes, too much standardized testing, and the perceived capitulation by Democrats to the education agenda of influential foundations and interest groups. But above all, the teachers took aim at their city's mayor, a testament to their frustration with his leadership of the schools, which the mayor controls under authority granted by a 1995 state law. The Chicago district has a history of contentious labor relations, but the strike was the first by the city's teachers in 25 years.