Bottlenecks and lever points of learning

It may be difficult to see bottlenecks and lever points in the classroom, because one characteristic of classrooms is that all students learn and do the same things, leaving the teacher as the primary bottleneck/lever point. What if the classroom became more like an ecology in which different students occupied different niches? We see this sometimes when introducing technology and learning that one or two students are already proficient, we rely upon them to help the other students and us. But that remains a very non-diverse ecology. Project-based learning tends to have different students be responsible for different aspects of a project. One source of bottlenecks could occur in the flow of information between different parts of the project. But that seems to be a bottleneck for completing the project but perhaps not so much for learning. Bottlenecks, and simultaneously lever points, for learning would involve key concepts that are required before others can be learned, or at least would facilitate later learning. And so, we're back to building blocks and Davydov's germ cell concepts. What are the germ cell concepts for writing?