CHICAGO (CBS) – The Chicago Teachers Union filed an unfair labor practice complaint Wednesday against the Chicago Public Schools system as a potential Sept. 10 teachers strike looms.

The CTU accuses CPS with canceling longevity pay hikes and sick leave benefits, among other moves, while both sides are involved in collective bargaining. The CTU filed its complaint with state labor officials and asks for them to step in.

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Union officials and school administrators continue to try to hammer out a new labor contract to avoid a teachers strike. Union delegates voted last week to strike as soon as Sept. 10 if a deal doesn’t happen.

So, what are the big issues on the table? CBS 2’s Derrick Blakley reports.

The school board is offering a raise of 8 percent over four years, but the Chicago Teachers Union wants more. The CTU also wants call-back rights for laid-off teachers because a number of schools are expected to be closed next year.

“There’s no question that the issue of job security would rise to the top in this contract. The handwriting is on the wall,” says Terry Mazany, a former interim CEO of the Chicago Public Schools system.

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The CTA is already losing jobs to the growing charter school system, which is staffed with non-union teachers.

Meanwhile, CPS simply has too many buildings and too many classrooms for the number of students. Enrollment peaked at about 500,000. It’s now at about 400,000 — 20 percent less.

Mazany says between 100 to 150 buildings could be considered “excess capacity.”

With consolidation, 100 schools could be closed, with perhaps 10 percent of teachers – 2,600 jobs – cut.

The union wants principals to hire only laid-off teachers for any new jobs. CPS says no deal.

“We need to make sure they get the best possible teachers and give the displaced teachers a fair chance, but not an exclusive chance,” Says Barbara Radner of DePaul University.

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CPS is already planning which schools to close or consolidate. It’s a budgetary necessity, and a political nightmare.