Mark, in my view — which I know is shared by many I talk to — teacher development is key. The interesting models I have seen acknowledge how much of teaching is about context. In other words, something that works with a class in rural Bangladesh won’t necessarily work with a class in Queens, even though both are learning fractions. Really interesting models treat the activity of teaching as “action research” — sharing and understanding what constitutes good practice, exploring how it needs to be adjusted in different contexts, and discussing these with peers and mentors. This is of course easily facilitated by common free digital tools, although there are some organisations working on dedicated solutions. Jim Knight may have more to add here.