The union and its allies say Friday's action is an attempt to draw attention to their fight for a new contract and better funding for a school district "on the verge of financial collapse."
Officially, the union says it's protesting an unfair labor practice after CPS announced in August that it would no longer pay salary increases based on length of service or educational attainment.
The walkout also comes shortly after schools CEO Forrest Claypool announced teachers and staff must take three furlough days to help CPS, which faces a $1.1 billion budget deficit, to manage cash flow.
Union members say they'll never get a fair contract unless the district has additional revenue, and they blame Mayor Rahm Emanuel, Republican Gov. Bruce Rauner and other officials for a multibillion-dollar budget crisis that's led to steep cuts.
In February, a union bargaining team rejected an offer that CPS said provided pay raises and guaranteed job security and met other union demands.
CPS and Rauner call the one-day walkout an "illegal strike," noting state law says the union and the district must exhaust a series of steps before teachers may strike over a contract, and that hasn't happened yet.
CPS is opening more than 250 "contingency sites" at schools, parks and libraries, where they say students will be "safe, fed and engaged" for the day.
