It would need to be something that's carefully designed, so that the teacher is able to differentiate. But I wouldn't just automatically dismiss it as a bad idea. I think it's a bad idea only if they do it wrong, rather than the way it was intended. My preference would be that schools ability-group. So if my kid in first grade is ready to go to third grade math, they put him in a group with other kids learning third grade math. But it's not happening and probably never will, so the flipped classroom at least sounds like an improvement over what is being done now, where DS is sitting with the rest of his class on the floor writing problems like 7-2 on his whiteboard every day while the teacher lectures.