First of all, you should never leave out Jazz music all together. April is Jazz Appreciation Month; that’s when I cover it. In General Music, I have most grade levels studying Classical Music through March, so adding Jazz creates opportunities for compare and contrast.
To your question: Have the students listen to a piece of music – any random melody (or one with which they are already familiar). Have them sing the melody on solfege or with lyrics or syllables (doo, dah). Ask the kids how they would make changes to that melody. Off the top of their heads, what would they change to make it their own melody. Explain that they could drastically change it – this is THEIR version!! Have the melody on a projector or written on the board. Write the students’ responses onto the score on the board. Have all understand what was done and how the original melody became that. Then all sing it again with the changes; have the responding student direct if they’re comfortable. Explain that when musicians improvise on a melody they do all of this on the spot in their heads.
Students must feel comfortable with their environment and the material / instrument (voice, glock, Orff instrument) before they’ll improvise. Explain to the class that no matter what happens in performances / improvising or what it sounds like they are to clap politely for their classmate and be supportive. If their classmates laugh they’ll never try it again. Improvising is risky for kids, yet once they get used to it, they’ll love it.
Play them some of the masters of scat and improv – Louie Armstrong, Ella Fitzgerald and such. Get them to hear the material, not the age of the individuals and the performances (not exactly recorded digitally).