Teachers in Jersey City went on strike Friday after they and the school district failed to reach an agreement over the cost of health insurance.

Members of the Jersey City Education Association, who teach 28,000 students just across the Hudson River from Manhattan, had been working under an expired contract since Sept. 1. The union also represents other school employees like paraprofessionals.

The strike was called on Thursday night after negotiations between the union and school board failed to bring about a deal. The two sides have met to negotiate more than 20 times. On Friday afternoon, a judge ordered the teachers to return to work on Monday.

“My 4,000 members have been disregarded and disrespected by the board,” Ronald Greco, the union president, said in a statement. “They are outraged by the board’s behavior, and they refuse to give up or accept it. My members are fighters. They fight for their students every day.”