Here is an excellent article that explains clearly why the strike was necessary. The article appears in the newsmagazine called F, published by the Art Institute of Chicago, a venerable institution.
Mayor Rahm said it was a “strike of choice.”
In this sense, he was right. The teachers could have just kept on teaching under dismal conditions for students, or they could strike and demand better conditions in the classrooms and schools for the students and teachers.
He chose to defend the status quo. The teachers said “no” to the status quo. They said the status quo was intolerable. And that is why they went on strike.