This warm-up for your older students is a partner song. The first time through, part one sings. The second time, part two sings. The third time, they sing together.
The focus of this warm-up is staying on pitch. Because it is mostly slurred, the interval movement is challenging. Singing in half-steps is also a good workout for those developing vocal cords. If you accompany this on the piano, or do it a cappella, you can alter it for more of a challenge. For example, you could progressively speed it up, or you could modulate it. (from Music K-8, Vol. 13, No. 5)