At Turion's request, I'm writing out a few ideas for conducting improvisation classes. If using my suggestions, they should be approached and reinforced cyclically, as they all support one another. For the sake of brevity, I'll provide one example for each.
- Listening / "Talking"
- Call & Response
- Response & Variation
- Variation & Extension
- Knowing Your Instrument
- Thinking Creatively - imposing limitations
Listening / "Talking"
Just as in a real conversation, you can't just talk whenever you feel like it, the same is true for music. If you're constantly interrupting who you're talking to, no one will want to talk with you. Same for music.
Idea for class
Give your students a prompt; they can only speak when you point to them; you gradually get faster until it falls apart. You can extend this activity by having a student be a pointer or by having them take turns with no one assisting / pointing. This activity can also be extended by using instruments / body percussion instead of voices.
Call & Response
Brainstorm situations where somebody might respond to you (saying "hello", a phone call, waving to a neighbor, etc). A response can be a direct quote or a related acknowledgement of what people just heard.
Idea for class
Divide into groups of two, have students practice listening to a statement from their partner, and try repeating exactly back to the other with the same vocal inflections. For a group, play "telephone" by whispering a word or phrase in a student's ear. They must continue this all the way around the circle until it gets back to you. If it changes, you get a point, if it's the same, they get a point. Activity is easily extended by substituting speaking for instruments with the same concept - you can even still have them play telephone.
Response & Variation
Very similar to the previous technique, the only difference here is that instead of reproducing the sound exactly, students add a tiny variation to it to make it unique. The same activities can be done.
Variation & Extension
Again, a graduated technique. Now, instead of just providing motivic variation, students are also tasked with extending their ideas through repetition.
Idea for class
Depending on ability, a great piece for structured improvisation would be In C by Terry Riley, or some pieces by James Tenney.
Knowing Your Instrument
When improvising, your instrument should be an extension of you. If you're limited by your knowledge of your instrument, you'll be limited in your ability to improvise.
Idea for class
Spend some time each class with students just fiddling around on their instruments. Encourage them to come up with five or ten unique sounds. Even if they only make one unique, new sound, that's still progress. Alternatively, you can give them a goal for more structure: "I want you to create a squeaky sound". This activity can be extended by having them create soundbanks, choosing 2-3 sounds, and then having an improvisation session where they can only use those sounds.
Thinking Creatively - imposing limitations
One of the difficult things about improvising / creating music is that there are simply too many options to choose from. Imposing limitations gives people something to push against, and pushing against something is where creativity comes from - you're forced to think differently.
Idea for class
Your options for limitations are endless. You can limit the time people play, the sounds they can make, their dynamic, the number of people who can play, the instruments they can use, or any number of other variables you want to influence. Remember that for students, structured improvisation is almost always more successful than improvisation that is completely free.
Good luck.