American public elementary and secondary schools spend about $20 billion a year on what is called professional development — helping teachers do their jobs better. Many teachers will tell you much of that is a waste of time and money.

Now, three former teachers involved in training have discovered an important reason. Teachers are rarely given time and opportunity to practice what they have learned.

“Practice Perfect: 42 Rules for Getting Better at Getting Better” is the new book by Doug Lemov, Erica Woolway and Katie Yezzi exposing this flaw in teacher training and the way most of us learn any complex skill. Professional athletes know the value of repeating moves again and again before the game starts. Michael Jordan was famous for how much time he spent practicing in the gym, even after he became a superstar.

But the rest of us either don’t see that or, as in my case, are too lazy to make the effort.

For teachers, the authors of “Practice Perfect” write, pregame repetition is crucial. “If a teacher’s performance during a given class is less than what she wanted, she cannot get it back,” they say. “She cannot as, say, a lawyer working on a contract might do, stop in the middle of her work and call someone to ask for advice. She can’t give it her best shot and then, as we are doing as we write this book, go back and tinker and revise and have the luxury of being held accountable for a final product that reflects actions taken and reconsidered over an extended period.”

“If we asked a roomful of teachers how often they practiced what they did in the ‘game,’ that is how often they rehearsed the questions they’d ask or the way they’d start class, most would look at us funny,” the authors say. “Teachers listen, reflect, discuss and debate, but not practice.”

The authors learned this only recently after analyzing the results of a study of great teachers in high-poverty public schools, reported in Lemov’s previous best-selling book, “Teach Like a Champion.” The teachers with the best results “were often the most likely to focus on small and seemingly mundane aspects of their daily work.” The authors liken this to legendary basketball coach John Wooden, who went so far as to teach players how to put their socks on correctly. But the insight did not immediately illumine the importance of practice.

As part of the Uncommon Schools charter school network, the authors began to train their teachers with video clips of skilled instructors performing a given technique. The trainees discussed and analyzed until they were sure they understood it, then moved on to the next technique. They left the class full of confidence, but when surveyed three months later were not so sure. Something often went wrong when they tried the techniques in class.

The authors realized their trainees hadn’t practiced. It was the equivalent of trying to learn a new backhand in the middle of a match at Wimbledon’s Centre Court.

They added repetitive exercises to their training workshops. Teachers played students so the situation would resemble a real classroom. Teachers still had trouble getting it right. The real-world situation was too distracting; the authors dialed down the student disruption so their trainees had a chance to do the technique correctly several times. Once it became automatic, they could handle unpredictable moments.

In their real classrooms the new skills began to work. Teachers not only implemented the techniques but also began to innovate and adapt them in new ways. The authors know this because they made and studied videos of the teachers at work, one of their favorite tools.

In the future, schools will still often spend big money on training teachers in ineffective ways. Learning to practice, this book vividly illustrates, takes time and effort, trial and error. It won’t happen tomorrow. But even a small movement in the direction of more practice will reap benefits, in teaching and many other things we do.

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