CHICAGO (CBS) — Chicago Public School teachers walked off the job for the first time in 25 years.

What are the key sticking points in the negotiations?

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SALARY: The two sides say they are actually close on this issue. The school district has offered increases of 16 percent over four years. The union is concerned about the rising cost of health benefits.

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JOB SECURITY: The union wants to ensure that there is a system to recall teachers who have been laid off. They are concerned about future school closings. The district has proposed giving laid off teachers a shot at new schools if there are openings.

TEACHER EVALUATIONS: The union is concerned that the new system is too complex and relies too heavily on student test scores. The school district said the evaluation system was created with teacher collaboration and should not be part of negotiations.

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WORK QUALITY: The union wants stronger rules on student/teacher ratios and is also demanding a plan to install air conditioning at schools.