That sounds like a very...odd proposition. I doubt the guy will give you any grief about it, but don't make up a "my wife wants it back"/"the kids miss it" line.
Just lay it on him, as long as you said in the beginning "here's how to care for it" and added a "any questions or concerns just ask" then just lay out your concerns, and what you want to happen. If you're in the same school, make a habit of stopping by DAILY to check on the snake. Feed it yourself, if you feel do inclined.
I personally wouldn't mind lending out a snake of my for educational purposes. I mean, my wife is 3/4 the way through a bachelors in teaching for high school biology. We've already discussed me displaying some of my reptiles in her class room and I've made it clear that I will be micro managing every aspect; even if I'm there every day of the week. That's just the way I am about my reptiles. I have a room mate with me right now too, he's well informed that anyone friends he has over the snakes stay up.
If you really want the snake back in proper care, immediately, then just go get it. It's not his property after all. The snakes welfare is more important than a wow factor in a classroom.
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