I recently began a JH jazz band program in a small school. I have found great success with the Dean Sorenson First Place for Jazz books. There are published books for mixed instrumentation, the pieces progressively increase in difficulty, and it does a good job of introducing improv to young jazz musicians. It also devotes decent attention to the rhythm section.
The "Effective Warm Ups for Developing Jazz Ensembles" by Mike Kamuf is a great free resource to get things off the ground as well.