Cycles for Change in St. Paul, Minnesota collects most of our bicycles through Bike Drives. Many of these have been coordinated in partnership with churches. We've found churches to be supportive of our work and focused on social justice. We've worked with a couple smaller churches and collected 20-30 bikes, and we also partnered with a much larger suburban church about 2 years ago and collected 150 bikes - which was almost more than we could handle. I highly recommend collaborating with churches for Bike Drives. In my experience, we've connected with a "social justice ministries" committee, rather than a youth doing service for confirmation, as Rich mentioned, but I'm sure both are opportunities to explore.
Other groups we've worked with to coordinate Bike Drives include: business, including REI which has been an amazing partner, doing bike drives at each of their three metropolitan stores; a local Lions Club, which brought in 90 bikes; Eagle Scouts, who have done some very successful bike drives.
This is from a few years ago, but Working Bikes Coop in Chicago had a pretty incredible operation coordinating dozens of Bike Drives and collecting bikes from throughout the Chicago metro area. Jason Tanzman