CHICAGO – For the first time in a quarter-century, thousands of Chicago teachers walked off the job Monday, escalating a bitter contract dispute over evaluations and job security and forcing parents to scramble for somewhere to send idle children.

Both sides went back to the bargaining table around midday, hours after the walkout began when the two sides failed to agree on a contract before a midnight deadline. The strike affected nearly 400,000 public school students and their families in the nation’s third-largest district.

While negotiators said they had made progress on salary and a longer school day, they remained divided on a host of other issues.