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," the new book by Doug Lemov, Erica Woolway and Katie Yezzi, exposes 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."