Simmering tensions at Chicago public schools rose a notch last week with the announcement that school employees would have to take three unpaid days off this school year.

Chicago Public Schools CEO Forrest Claypool said that the move to lop off one day in March and two days in June is an effort to “chip away at our budget gap” and would save $30 million.

Karen Lewis, Chicago Teachers Union president, responded that adding the unpaid days to a 7% giveback on teacher pension contributions the school district is already requesting in contract negotiations would make for an 8.6% pay cut overall.

And so the Chicago Teachers Union, the third largest in the country, said the system’s move “all but assures” that teachers will go on strike, perhaps as soon as April 1.

The Chicago fracas is emblematic of the grim state of public school/teacher relations across the U.S., a relationship that resembles the exhausted late rounds of a heavyweight boxing match.

Public school teachers’ unions, once able to guarantee their members ever rising wages but now weakened by years of attacks from elected officials, are fighting a rearguard battle with financially insolvent school systems—battered by sagging funding and higher enrollments—to retain pay and benefits won long ago.

The Chicago school system, which employs 37,400 workers, is facing a $1 billion shortfall—larger than most school districts, but far from unique. Major school systems across the country are being rocked by budget gaps and other problems.

In September 2012, Chicago teachers took to the picket lines for a week in a standoff over job security and teacher evaluations. The start of the current school year in Seattle was delayed last fall because of a weeklong teachers’ strike over wages. And this January, Detroit teachers staged a “sick-out” over class sizes; absences were extensive enough to force the school system to close two-thirds of the city’s public schools.

While strikes like these are widely criticized for hindering children’s education and inconveniencing working parents, they can work. In Seattle, teachers won a 14.3% pay raise over three years, as well as other concessions including more input over standardized tests. And in the 2012 strike in Chicago, after 350,000 students stayed at home and delayed the start of the school year, the teachers won some concessions.

But today, that dynamic is changing. Fewer workers are unionized than in the past, and governors like Wisconsin’s Scott Walker are laboring to lower the levels even further, which would hinder the ability to strike for wages or working conditions.

Not to mention that many school systems, like the one in Chicago, are broke.

Illinois Gov. Bruce Rauner, a fiscal conservative, has said that bailing out the school system would require crippling increases in property taxes for local residents. He is backing a plan to have the state legislature in Springfield pave the way for the Chicago school system to declare bankruptcy. That would, in turn, eliminate the system’s obligations to its current employees.

For his part, Chicago schools CEO Claypool has urged state legislators to provide more money and address the “unequal funding issues that hurt districts like ours across the state.” Chicago’s system can’t slash funding enough to come up with a solution, he noted. (The average Chicago teacher’s salary is about $75,000 a year.)

It was in this tense atmosphere that, last December, 88% of Chicago Teachers Union members voted to authorize a strike. At the time, it looked like hundreds of teacher layoffs were in the offing. Last month the tension seemed to ease when only 62 layoffs were announced (17 were teachers). But amidst an atmosphere already made raw by student shootings and accusations of police brutality, the announcement of the furlough days ended the honeymoon.

For now, the first furlough day—a money-saving tactic that was last used in Chicago in 2011—will be March 25. The next two furlough days are set for late June, but a teacher strike that would effectively end the school year may well be underway before then.