CHICAGO (CBS) — Teachers at four schools in the northwest suburbs have gone on strike, after last ditch negotiations broke down overnight.

Classes at Prospect Heights School District 23 have been canceled for at least the next two days, after the union and the school board could not agree on a new contract. Teachers walked out around 12:30 a.m. Wednesday, starting the first teachers strike in the district’s history.

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Approximately 150 teachers walked the picket line outside District 23 headquarters on Wednesday, as they began a strike that – at this point – appears as if it will last at least two days.

Eleventh hour negotiations broke off early Wednesday morning, with both sides still far apart. The main sticking point has been teacher salaries.

“We put across a proposal to the board, at which time we were anticipating a return proposal. They responded with no comments, no response, which then told us that our night was over,” said Bob Miller, president of the teachers union, the Prospect Heights Education Association.

The union said teachers wanted to return to the negotiating table anytime Wednesday, but the school board said it would not be available, so classes will be cancelled Thursday as well.

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The teachers have been seeking pay increases of more than 4 percent per year in each of the next three years. The district has maintained the teachers’ demands are excessive, and has offered raises of about 3 percent per year.

Miller said the union is seeking better salaries so good teachers don’t leave for other school districts.

“[District 220 wants] to hide the fact that we do have the lowest retention rate in the surrounding area at 80 percent,” he said. “We have the second-lowest average teacher salary in the surrounding area.”

Miller said the district could pay the raises teachers are seeking by dipping into its $8 million reserve fund. District 23 Supt. Debbie Wilson said there is some wiggle room to negotiate pay, but dipping into the district’s reserve fund would not be a good thing to do.

“Illinois is in a rough shape. You are looking at the governor talking about property tax freeze. You’re looking at the governor talking about pension shifts. So you have to be able to respond to that,” she said.

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The district serves portions of Prospect Heights, Arlington Heights, Mount Prospect, and Wheeling. About 1,500 students at three elementary schools and one middle school in Prospect Heights will be staying home as their teachers and school administrators joust over pay and benefits.