I love this idea that I heard of recently that was done in a friend's primary for music time. The kids in primary LOVE stickers and are motivated to sing if they know they can earn a sticker of any kind. So why not get some round stickers, maybe happy face stickers but call them "MEASLES". And the kids can earn a measles!
"It's measle motivation"!
Pass out a strip of a dozen stickers to each of the teachers. After each song, tell the kids "Your teachers are going to give you the MEASLES if your singing is great!" Then just lead the songs like you normally would, showing the visual aids, or just directing.
You can point out to the teacher somebody YOU think deserves a measle sticker, but it is really going to be their jobs. Have them put it right on the kids' faces. It is super fun. That way each class will be motivated and recognized.