The cheaper Hovabators require a lot more "fiddling" with the controls than a Hovabator Genesis or Brinsea. And yes, I have worked with all of them. If a kid bumps the controls on the old-style wafer thermostats, it can very easily kill all the embryos before someone notices. It's much harder to mess things up with the better models.
Check out Amazon for Brinsea incubators, and go to Cutler Supply for the Genesis. I don't think you can go wrong with either, depending on how many eggs you want to hatch. For a classroom project, where cranking out a lot of chicks is not the point, I would start by looking at the Brinsea Mini Advance (about $180) and if that doesn't have enough capacity, then move up from there.