Teachers in Evergreen Park late Monday night decided to strike after failing to reach an agreement with the district on a new contract.
The action means that about 1,900 students in five schools won't have class until a deal is reached.
A union official said there was only little movement in Monday's talks and said the two sides remain far apart when it comes to insurance coverage.
The district has posted a contingency plan, as well as information about its offer to teachers, on the district website at d124.org.
There was a celebratory mood earlier in the day as about 300 teachers gathered outside Central Junior High School chanting about their desire for a new contract.
Insurance, retirement benefits and compensation are the three main sticking points. The two sides are nearly $2 million apart in their negotiations, according to school officials.
"It frustrates me, and I'm sad for the kids like in any strike situation. The teachers want and deserve a fair settlement, and the district and the board and I have a responsibility to be good custodians of the taxpayer money. Sometimes those two issues don't see eye-to-eye and it's just unfortunate," said Supt. Robert Machak. "I don't want to see kids put in the middle of an adult problem."
Teachers said they just want to maintain salaries -- not increase them -- and said that's doable because the district is sitting on a $16 million reserve fund.
"They're not asking to make gains here. They're just trying to hold onto what they have [and] keep this district competitive in a district that's very financially stable," said Dave Comerford with the Illinois Federation of Teachers.
Both sides have been meeting weekly since April.