If I'm understanding your story correctly your math teacher did the opposite of what the person I responded to said - he separated out the high performers, creating an environment that allows them to thrive and achieve high results. This is great, but what about the ones who weren't picked into the high performing class?

There is nothing saying you can't have good classes for both high and low performers, but with schools that have limited resources it often works out to a poor medium where no one really thrives (hence the caveat of 'in my experience' in my comment).