Thanks for the support & advice. Just so you know, it's been on my mind all weekend - telling the kids - and I think I'm going to do it Monday just to get it over with. I'm not sure why I think it will go so badly. I do worry that they'll "check out" but that's their dumb mistake if they do - I'm still their teacher until the end of the semester, so I'll still be taking grades!!
It's just that their "good behavior" party will also end up being my going-away party.
Thanks for the support :) They've got a test tomorrow, so I'll just break the news after the test, maybe 5-10 minutes before the end of class.