IN THE 3½ years that John Deasy led the public schools in Los Angeles, student test scores climbed, suspensions dropped and the high school graduation rate rose to an all-time high. So one would think that the school board would do everything in its power to keep Mr. Deasy. Instead, it pretty much showed him the door — a vivid demonstration of how political interests trump results when it comes to America’s broken schools.

Mr. Deasy resigned as head of the Los Angeles Unified School District on Thursday after reaching an agreement with a school board that had made no bones about wanting him out. The seven-member board was to meet this month to review his performance, but there had been reports the board had already authorized its attorneys to discuss a departure agreement. Ramon C. Cortines, who preceded Mr. Deasy as superintendent in Los Angeles and also did a stint in New York City, was named interim chief. It’s not a prudent trade to cashier a hard-charging superintendent with a proven record of success in favor of someone who, at best, will be a caretaker.

Mr. Deasy always had an edgy relationship with the board (he came close to resigning last year), but things got worse because of stepped-up attacks from a newly radicalized teachers union and changes on the school board that made it less reform-minded.

Mr. Deasy, whose leadership of Prince George’s schools we admired, brought refreshing reforms to the sprawling system. That included breakfast in the classroom and tying teacher review to student test scores. Clearly, he made mistakes: He admitted being single-minded to the point of being bull-headed. What one writer called his trademark impatience caused him to so badly bungle a plan to equip every student with an iPad that it has been placed on hold and the bidding process subjected to scrutiny. But that misstep and the complaints about his style were mere pretexts for critics with an agenda that is not served by Mr. Deasy’s push for change.

Foremost among the reactionaries is the teachers union, United Teachers Los Angeles, which led the charge for Mr. Deasy to be held “accountable” by the board. The union has new leadership and is in the midst of contract negotiations in which there are differences on issues like teacher pay and evaluations. Another factor was Mr. Deasy’s decision to testify on behalf of plaintiffs who successfully challenged California’s archaic tenure laws.

But Mr. Deasy’s departure cannot be blamed on the teachers union. Its mission, after all, is to protect teachers’ interests. Mr. Deasy is gone because neither the school board nor the city’s political leadership were willing to give their support to a superintendent who made student interests his first priority.