Second, while it might seem better to wait on granting tenure, early decisions — not in the first year, but soon after — actually improve student achievement. That’s partly because stable faculties are better for students, but also because an attentive district knows a great deal about which teachers are good and bad after just two years, and waiting longer provides little additional information.

Finally, the freedom to fire experienced teachers is valuable only when dismissal rates are very high, say, 40 percent or more. And yet such rates come with costs: The risk of firing good teachers is high, and the impact on a school’s culture is detrimental to learning.

But with lower dismissal rates, marginal teachers are much worse than the average new recruit, and it is more important to get rid of them quickly than to get the decision exactly right.

Suppose you don’t want to fire more than 20 percent of teachers. You face a decision: Once you’ve fired the worst 19 percent and have two candidates who each might be the next worst, do you decide early, realizing that you might make a mistake, or do you wait to be sure you’ve got it right? In this case, the costs of waiting outweigh the benefits, and you should do it soon. Tenure laws can usefully tie your hands, forcing you to do that.

In short, while the notion of “clearing the stables” of bad teachers seems attractive, it is almost impossible to get right in practice. No conceivable system can eliminate all “grossly ineffective” teachers, and efforts aimed at doing so can do more harm than good.

Everyone agrees that closing the achievement gap should be a high priority. But the remedy should fit the problem.

The lack of effective teachers in impoverished schools contributes to that gap, but tenure isn’t the cause. Teaching in those schools is a hard job, and many teachers prefer (slightly) easier jobs in less troubled settings. That leads to high turnover and difficulty in filling positions. Left with a dwindling pool of teachers, principals are unlikely to dismiss them, whether they have tenure or not.