An old friend of mine who had worked as a school principal once told me that the biggest impediment to improving the quality of instruction at his school was that he couldn’t fire lousy teachers.

Now, I don’t know if the teachers Michelle Rhee, the former D.C. chancellor of D.C. schools, whom I admire enormously, fired were lousy, but I do know she couldn’t fire them:

The first 75 teachers who former Chancellor Michelle Rhee fired must be given about $7.5 million in back wages and offered positions with D.C. Public Schools, an arbitrator ruled.

“The [termination] process used in this case was so devoid of due process as to be arbitrary and capricious,” arbitrator Charles Feigenbaum said in his verdict favoring the Washington Teachers’ Union, which has been fighting D.C. Public Schools officials over the July 2008 dismissals for more than two years.

Rhee fired 75 first- and second-year teachers after asking principals to recommend recent hires for dismissal. D.C. Public Schools officials told teachers that they couldn’t appeal because they had yet to earn tenure, but did not disclose the reasons for their dismissals.

“We have an important decision that shows you what termination was like during the Rhee-Fenty administration: It was wrong,” union President Nathan Saunders said.

Saunders estimated that with an average annual salary of $50,000, each teacher would cash in on $100,000 — costing the government-run school system $7.5 million while the city faces a $545 million deficit in fiscal 2012.

A spokeswoman for Mayor Vincent Gray said that with “the budget crisis that we’re in, at this point there’s no extra money,” but that the District would follow the law.

The villain here is the tenure system-Rhee had to limit her firing to teachers who had not yet earned tenure and were thus not invulnerable. Well, as it turns out, they were invulnerable.