This file photo shows an LI classroom on March 23,...

This file photo shows an LI classroom on March 23, 2011. Credit: Newsday / Karen Wiles Stabile

The teacher tenure war that’s been simmering across the nation for years has really begun to boil over, and the latest hot spot is Philadelphia. Soon, hopefully, the unassailable privilege of long-serving teachers to keep their jobs no matter how poorly they perform will be cooked beyond recognition.

Thus far, it’s students, and often the poorest students, who’ve been burnt by the teacher-protection laws.

Philadelphia has 125,000 public-school students, a miserably ineffective school system that’s been under state control for 13 years, recurring budget challenges and teachers unions fighting change. What’s at stake is one of the core perks of a unionized teacher position: employment decisions based entirely on the previous length of that employment. It’s a more precise metric for judging teacher effectiveness than height, weight or hair color…but not by much.

District officials laid off 3,800 teachers last year, although about 1,600 were called back after some emergency budget measures were put in place. The layoffs were done entirely by seniority, as the teacher contract and state law demand. For the callbacks, though, criteria other than length of service were considered, which led to an avalanche of grievances.

Because the city’s schools have been placed under state control, the state law that lays out he privileges of seniority can be suspended. What’s less clear is whether the teacher contract that enshrines seniority can be bypassed, an issue made more complex by the fact that teachers have been working without a contract in place since 2013.

And it’s not going to get clearer anytime soon, as Pennsylvania's highest court refused to take up the question.

Now, the city is almost certain to have to undergo more staff cuts, and is telling school administrators they can bypass seniority to retain teachers with less time in the system if they have a “compelling reason” such as showing an educator had a “measurable positive impact on student achievement,” according to a story in Monday’s Wall Street Journal.

The teachers unions are ready to fight the erosion of seniority like it’s a life or death matter, which for unions, it is.

This confrontation comes just weeks after a judge in California wrote a decision that essentially blew up that state’s extremely labor-friendly teacher-tenure law, ruling in part that it disproportionately affected the educations of low-income students by letting poor teachers, who are too often parked low-income schools, stay in place.

In New York, too, the game is afoot and the stakes are high.

Thursday, an education-advocacy group called New York City Parents Union filed the first challenge to New York’s teacher tenure laws in State Supreme Court on Staten Island. The suit is heavily reliant on the same type of reasoning that prevailed in California. It’s been reported that only 12 teachers in New York City were fired for poor performance between 1997 and 2007 because it is so extraordinarily expensive and and time consuming to get rid of tenured teachers: we’re talking hundreds of thousands of dollars and year after year of procedural delays.

It’s not easy to fire tenured teachers on Long Island or anywhere else in the state for poor performance, either.

Students are best served by the best teachers. In some cases these teachers are the longest serving, in others they are not. The battle over performance standards for teachers in New York is, at its core, a battle over the idea that differing levels of merit and effort will bring about different outcomes, pay scales and futures for members.

That’s a future the unions oppose. It’s a future students have to have. And it’s a future that the courts are likely going to be more and more willing to impose as economically disadvantaged students are shown to be the ones saddled with ineffective teachers who can’t be fired.