(CBS) — Chicago’s teachers could go on strike as early as next week, unless its union and the district reach an agreement on a new contract.

The city is offering the teachers a 16% raise over a five-year contract, but teachers propose a 15% raise over three years.

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The union says the issues go beyond money. It has set a possible strike date of Oct. 17.  

The last time teachers walked off the job in Chicago was in 2012. That action lasted nine days.

The teachers say they want smaller class sizes.  CPS serves about 360,000 students in 165 high schools and 477 elementary schools.  CPS says it gives principals discretion on teacher staffing. CPS policy sets limit on class size in elementary school at 28 students; in middle and high school, it’s 31 students. The union claims some classes have as many has 40 students. Many of those are in poorer city neighborhoods, according to its own analysis.   

The teachers’ union is demanding elementary student limits to be set at 24 and middle and high school students at 28. The district provided the following example that would provide 22 teachers for an elementary school of 600 students.

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The teachers union is asking for more nurses and social workers. CPS has included $10 million in its 2020 budget to hire 95 new employees. The union says it wants that staffing written into their new contract.

The overall $7.7 million budget would include $6.18 billion to operate the schools and $821 million to upgrade facilities.  The remaining  $700 million would go to pay off school debt.

CTU is also asking for more librarians. The 2020 proposed budget does not specify any money for new hires, but it does propose expanding access to digital e-books and plans to add 600 new books to the CPS Virtual Library.

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Mayor Lori Lightfoot says she is committed to reaching a deal, and said there are no plans to make up lost days in event of a strike.