From Karin Chenoweth at the Britannica blog:

Everyone in a school knows that some teachers are effective and some aren't, but in most schools there is no organized way to ensure that students who get weak or bad teachers still learn what they need to learn. That's not his fault, or the fault of any individual teachers who work hard; it is the fault of the way schools have been organized for generations.

and

[T]he essential issue at the heart of the controversy is whether schools should be asked to organize themselves to have student learning be at the center of all of their activities. I say they should. But the fact is that relatively few schools have done it.