"The kids will be so disappointed"

What a joke, and no freaking excuse for a second chance at neglect. I'd be livid if something like that had happened, especially after the lengths you went to make it as easy as possible. I bet the melting plastic fumes didn't help the RI at all

It would be absolutely in the realm of politeness to just say "I don't wish to put my animals back in your care after the state I found my snake in." Maybe offer to bring your animals in for a show and tell sort of thing. If he wants a snake in the classroom, he can buy one himself to kill (or hopefully learn how to do it properly and see why you wouldn't give him yours, who knows)