"How do the civic purposes of education fit in this model, which is so focused on differentiation and students moving at an individual pace?"
This is an important question to address because, like Gary has pointed out, a school like the one I described in my post has the potential to become a place where we see fragmented, isolated learning--not exactly the type of school I argued for. In this scenario students would become self-absorbed learners with no real concern for using their education to make a difference in their community.
But, think that a combination of two things could keep this from happening.
1. Humble learners: Humility--characterized by a recognition that one's knowledge, perspective, and experiences are limited--would seem to draw a student to other learners in an attempt to glean from their studies and experiences. Even if a student is pursuing their own learning, this element of humility would seem to engender a curiosity and desire to see how their learning fits with the others in their learning community or classroom. How does what my classmate is studying connect to my learning? What might I learn from her that could advance the learning I am doing in my realm?
In a related way, humility would also seem to lead to a gratitude for the learning one has acquired. When a learner is grateful they seem to want to share their learning and use it to improve the people and things around them. A humble learner would then look for opportunities to apply their learning towards grappling with authentic questions and problems in their environment. For example, A high school soccer player studying human anatomy and basic principles of sports medicine might seek out opportunities to help teammates experiencing injuries.
Teachers and schools should explore ways of helping students to develop this attitude of humility in their approach to learning. At first glance it seems inefficient and disconnected from their purposes. But, it could pay big dividends in terms of the way learners interact with each other and their communities.
2. Teachers who know how to integrate students' learning in meaningful ways. Gary's question has helped me realize that good schools don't just allow learners to pursue individual learning goals. At some point, in the process they also bring learners back together and engage them in meaningful collaborative work. In my mind's eye I see students invidividual learning equipping them to serve as "expert stakeholders" or "consultants" on group projects that require each group member to bring their learning and apply it towards developing a solution to a real problem that has been presented by the instructor. In this way students personal learning goals are honored and validated, but they are also shown how their individual learning can be joined with others to produce powerful results. In these classrooms an instructor's role is to, first, help students pursue their own learning and, then, to craft and identify problems/tasks/projects that bring groups of students together in a democratic way.
In short, while differentiated and self-paced learning might appear to run counter to the civic purposes of learning, if facilitated in the right way it may have the potential to be even more powerful than the type of civic and democratic learning we see in more traditional classrooms.