Ehh.... I didn't have great luck. 2 out of 12/18, and I lost one after hatching. But mine were araucana that came from Texas (so across the country) so between the distance and the breed, they had a few struggles already against them. My male I have from the hatch is super great though.
Fill a paper (plastic would probably work too) full of ice and then add just enough cold water to make it wet. Stick the probe in and see where it lands (should land smack on 32F).
If it doesn't, there should be a nut looking thing attaching the face to the probe and that's what you'll turn to...
Thats what I was thinking. It was also metal, so theoretically you could always just hose it out when cleaning and then a quick rag wipe to dry if you needed to put chicks back in right away. Maybe something to mention to the principal the next time you're at the school, just that you'd be happy...