Last week New Jersey Gov. Chris Christie signed legislation overhauling the state’s teacher tenure policy. Previously, K-12 teachers were granted tenure after just three years and no meaningful evaluations. Now teachers will not be eligible for tenure before four years and only after earning ratings of “effective” or “highly effective” on their annual evaluations for two consecutive years. As the Wall Street Journal editorialized:

After two years of debate, the Garden State is scrapping the country's oldest tenure law—enacted in 1909—and for the first time tying tenure to merit. But ponder the status quo ante: If a school is going to hire someone for life, shouldn't it have been, you know, considering details like skill, talent and work ethic all along?…

The most important task now is ensuring that the evaluation system isn't watered down in practice. The board of education will define next year what hazy terms like "effective" mean, and school districts can either adopt the state system or devise their own model. Mr. Christie wants student test scores to account for half but says he's willing to whittle that down as low as 33%. The powerful New Jersey Education Association, predictably, lobbied successfully against including any official ratio in the statutory language.

The union no doubt prefers the current system, in which it is almost impossible to give even the worst teachers the hook. Over the last decade fewer than 20 teachers have lost tenure because of what the state calls "inefficiency"—i.e., for cause—in a process that can take years and cost more than $100,000 in legal fees. The new bill sends tenure disputes to arbitrators, not administrative courts, and caps the cost at $7,500 per case, which should mean fewer no-accounts and sexual predators in the classroom. …

Still, Mr. Christie and Newark Democrat Teresa Ruiz deserve major plaudits for overcoming what the Governor labels "inertia" in education. Tenure was designed to protect good teachers from arbitrary abuse, but over the years it has morphed into a fortress for mediocrity and union control. To adapt Dr. Johnson, the surprise is not that tenure reform is done imperfectly, but that it's done at all.

Other states endeavoring to improve teacher quality should pay close attention to the success of the Governor and Sen. Ruiz.