But at its heart, the strike is over the union's deep opposition to what it calls a "corporate reform agenda" that pursues a competitive or punitive relationship with teachers, rather than a collaborative one. Examples include blaming teachers and unions for educational shortcomings, promoting private but publicly financed charter schools, focusing on high-stakes tests and tying pay to merit.

CTU has instead pushed for smaller classes, enriched curriculum, better supplies and facilities, fairer and fuller funding (including the return of some public revenue long diverted into "TIFs" to subsidize developers), more counselors and support staff, respect for teacher professionalism, and a bigger say for teachers in their schools.

That clash puts the union at odds with CPS, the mayor and President Obama--whose education secretary, Arne Duncan, boosted the corporate-reform agenda as former Mayor Richard M. Daley's school superintendent. It also represents a more forceful rejection of such reforms than espoused by the national union, which nonetheless supports the CTU strike.