Thousands of teachers walked off the job in Chicago Monday, marking the city's first public school strike in 25 years.
Months of negotiations have fallen through between the teachers' union and the Chicago Public Schools District.
Salaries aren't the biggest issue at stake, but the two sides are hung up on issues like health benefits and a new teacher evaluation system.
"We must do things differently if we are going to provide our students the education they so rightfully deserve," Chicago Teachers Union President Karen Lewis said.
She added that, aside from benefits, the union is also concerned the new teacher evaluation system "could result in almost 6,000 teachers, or nearly 30 percent of our membership, being discharged within one or two years."
The walkout posed a tricky challenge for Mayor Rahm Emanuel, who said his main concern is keeping nearly 400,000 children now without teachers safe and occupied.
"My team is available at any time now to pick up where we left off, so we can get our kids back in the classroom," he said. "Our kids, the kids of Chicago, belong in the classroom."
Sunday, Emmanuel said he was "disappointed" the union decided to continue with the strike.
Some 26,000 teachers and support staff are expected to join the picket.
Meanwhile, 140 schools opened their doors Monday to provide breakfast and lunch to children who rely on free meals.