The union representing Chicago's public school teachers extended a strike, forcing tens of thousands of children to stay out of class for a second week. A majority of some 800 union leaders voted to reconvene on Tuesday rather than end the strike in the nation's third-largest school system, so that members could have more time to consider a contract deal that negotiators reached with city officials over the weekend. In an angry statement, Chicago Mayor Rahm Emanuel said he would seek a court order to immediately end the strike. "I will not stand by while the children of Chicago are played as pawns in an internal dispute within a union," he said. "This was a strike of choice and is now a delay of choice that is wrong for our children." Chicago Teachers Union delegates said that, following a "civil and frank discussion" on the proposed contract settlement, they "voted NOT to suspend the strike." The union said it will "allow two more days for delegates to take the information back to the picket lines and hold discussions with the over 26,000 members throughout Chicago." Issues raised in the contract proposal "were too important, had consequences too profound for the future of our public education system... for us to simply take a quick vote based on a short discussion," Chicago Teachers Union president Karen Lewis said. Emanuel claims the strike is illegal under state law because it concerns "issues that are deemed by state law to be non-strikable," and because it "endangers the health and safety of our children." A key issue in the strike is a new form of teacher evaluation that the union says relies too heavily on student test scores and could lead 30 percent of its members -- some 6,000 teachers -- to be fired in up to two years. Union delegates have the authority to end the strike, but its thousands of members would have to approve or turn down a new contract. The strike, the first of its kind in 25 years, is especially awkward for President Barack Obama because the union is fighting Emanuel, his former chief of staff who left the White House to run for mayor of Chicago. Unions are a key source of support for Obama's Democrats, and the party has fought to maintain the support of teachers even as the president pushes for major education reform that would give schools more leeway to fire poor performers.