Why do we neglect the rhythm section so often? Is it because we're not exactly sure what to teach them? Is it because ther are always much bigger "problems" in the horn sections? Is it because we can't afford to take so much time away from the rest of the band in order to focus on only 3 or 4 people? Yes, yes, and yes (if you are like me).
1. Teach Them How to Improvise a Bass Line
This is important for the bassist, of course, but also useful for the piano and guitar players. It is also worthwhile for every member of the band, so why not teach everyone the basic rules of creating a walking bass line? Once the rules are set down, and everyone has practiced some sample written bass lines (drummers can play time or use a vibraphone), have kids take turns improvising a bass line while everyone else plays a repeated riff or practices common comping rhythms. Keep the chord progression simple (only 2 or 3 chords), and you'll find the kids all enjoy the challenge.
2. Teach Them How to Play Different Chord Voicings
This is most important for the piano and guitar players, but everyone benefits from learning to play chords in different inversions and spacings. Start with a single chord, and help the pianist find two different effective voicings. Meanwhile the guitarist can be looking in a chord book or figuring out a different voicing for the basic voicing he has already learned. Meanwhile, the horn players are trying different chord inversions within their sections. When several minutes have passed, each section or individual demonstrates their two favorite voicings for the class. Then everyone plays the two voicings as a class on a rhythmic riff, vamping while the drummer lays down a nice time feel. Keep exploring different voicings with the piano and guitar players throughout the year, and help them understand that switching around their voicings helps them lay down a sweet groove.
3. Teach Them to Improvise the "Comping"
Most jazz charts include written rhythm section parts. However, competent jazz musicians NEVER play exactly what is written. They improvise their accompaniment (or "comping"). Is it hard to do? Yes. Is it too hard for young musicians? No way. But, it takes some understanding and some skills. This topic demands some description of each instrument separately, so bear with me.
Most written BASS lines are pretty decent, but the bass player should, as a minimum, strive to improvise the bass line in the solo section, since it is likely to be repeated many times, and allows the bassist to respond to the soloists and other rhythm section members.
Most written PIANO parts are also decent, and sometimes need to be played as written in order to match the rhythms and harmonies in the written horn parts. Like the bassist, the pianist should strive to comp improvisationally in the solo sections, being sure to alternate with the guitarist and to provide a complementary accompaniment to what the soloist is playing. During most other parts of a song, the pianist should explore different voicings and comping rhythms from what the written part suggests in order to create a nicer groove and develop her comping and soloing skills.
Most written GUITAR parts are too complicated for the knowledge and skills of the average guitar player. That is not to say that guitar players tend to be dumb, but rather that the parts are poorly written by composers and arrangers. Whereas young saxophone players can be expected to know every chromatic note on their instrument, young guitarists cannot be expected to know every chord and altered chord possible on the guitar (and a quickly accessible voicing). Typical guitar parts are written with far too many chords, and those that offer "suggested voicings" only offer badly-spaced voicings that are too far away from each other to be useful. Thus, a teacher must spend a little time helping guitarists to simplify the chord progressions and find useful 3 and 4-string voicings that are easy to switch between. A teacher also must help the guitarist play the right style at all times. Without all of these things, a guitarists comping will likely be detrimental to the sound of a jazz band, and is not prepared to comp correctly.
Most written DRUM SET parts are really bad - they either sound wrong when played precisely, or they are repetitive and boring. From the very beginning drummers should be taught to use the written part as only an outline of the song. They should be encouraged to freely add improvised rhythms on the snare and toms, to try using different cymbals than are notated, and to set up the band in a variety of ways. Especially during the solo section, the drummer should listen and respond to the soloist and help keep the groove comfortable and the energy level constantly building through each solo. The drummer should be playing live, improvised accompaniment all the time, even in performances (within certain limits, of course).
4. Feature the Rhythm Section in Concerts
It is common for many school rhythm section members to feel like their only job is to back up the more important horn players. In contrast, the rhythm section is THE MOST important section of the jazz band. Make sure these kids each have a chance to be featured in each concert. You can do this by giving them solos, extending written solos, adding vamps at the beginning or end of a song in which the rhythm section grooves and/or takes turn soloing, or choosing songs that feature them more. A little bit of attention goes a long way for getting more commitment and hard work out of your rhythm section.
If you want a jazz band "method" that has this kind of rhythm section training built into it, then Improv Pathways may be right for your band. The warm-up exercises, lessons, and activities provided in Improv Pathways will give your rhythm section the knowledge and skills needed to play and comp as real jazz musicians do. They will love learning these things, and you will love how easy it is to teach them with your entire jazz band actively involved.
Whatever methods you may use, I hope your rhythm section will be empowered to play and comp with the infusion of improvised spontaneity that jazz music demands.