Chicago Mayor Rahm Emanuel has moved for a court order to end a strike by thousands of teachers, as their walkout goes into a second week.
The city school district applied for a preliminary injunction against the Chicago Teachers Union (CTU) "to end the strike immediately".
The walkout, affecting 350,000 students, is over working conditions and job security.
Union members voted on Sunday to extend the strike for two days.
The delegates unexpectedly rejected an outline deal reached by union leaders on ending their action.
Monday's legal challenge by Chicago Public Schools said the strike was a danger to public health and safety, partly because thousands of public students rely on free meals for basic nutrition.
It also said the strike was a violation of Illinois state law that prohibits strikes except for wages and benefits.
A hearing to consider the legal complaint to force teachers off picket lines and back into classrooms was expected later in the day.
"State law expressly prohibits the CTU from striking over non-economic issues, such as layoff and recall policies, teacher evaluations, class sizes and the length of the school day and year," the school district said in a statement.
"The CTU's repeated statements and recent advertising campaign have made clear that these are exactly the subjects over which the CTU is striking."
The first strike of its kind in the city for 25 years, it has turned personal, with union leader Karen Lewis branding Mr Emanuel a "bully" and a "liar."
The Democratic mayor - President Barack Obama's former White House chief of staff - took the dispute to court after CTU delegates voted on Sunday against suspending the five-day strike.
The confrontation with a labour group that generally backs Democratic candidates is seen as an embarrassment to Mr Obama, in his home city and in the middle of a re-election battle.
Proposed teacher evaluations were one of the key issues cited as teachers walked out after lengthy contract talks stalled.
The CTU fears more than one in four public school teachers could lose their jobs and benefits if an evaluation of their performance based on students' test scores goes ahead.
Chicago, like many major city school districts, has been losing students to the suburbs and has a high percentage of children from low-income households.
A majority of the 800 or so union delegates in the nation's third-largest school district rejected a 23-page outline deal and extended the strike until at least Tuesday, when another meeting is scheduled.
The teachers' union leader had earlier said she would ask the delegates to suspend the walkout.
Mr Emanuel faced a $700m (£438m) school budget shortfall when he took office last year and he has said the contract agreed with the union would cost $295m over four years.
Parents have had a mixed response to the dispute.
"It's been very, very frustrating, especially knowing that they were close [to a deal]", Humberto Ramirez told the Chicago Tribune.
But mother-of-one Pamela Edwards told the Associated Press news agency: "I don't think they're wrong. The things they're asking for are within reason."