CHICAGO (CBS) — Sixteen teachers who claim they were fired from Urban Prep Academies charter schools for union activities staged a demonstration in the Loop on Friday, attempting to pressure the school to reverse their dismissals.

Chanting “bring our teachers back,” members of various unions and clergy rallied outside the office of Urban Prep Academies chairman Joseph McCoy.

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“I was fired for organizing a union. That wasn’t the official reason,” said Mathias Muschal, who taught English, and coached football and track, at the Bronzeville campus for six years, before he was fired in June.

Muschal said he was told he was being let go for throwing a pizza party for students and parents without getting approval from the administration ahead of time

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“I was told that that pizza party was indicative of my disdain for leadership,” he said.

Brian Harris, president of the Chicago Alliance of Charter Teachers and Staff, said just weeks before the firings, a majority of Urban Prep teachers voted to join that union.

“Urban Prep has to make things right; accept the fact that the teachers voted for a union. You know, they may not like it. It doesn’t matter. That’s the law,” he said.

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The union has filed an unfair labor practices complaint.