While it's ridiculous that she should have to "meow" to him, she should not have made the litter-box comment or "barked" at him as he left. That's where she made her mistake. She would not have been fired had she simply refused to "meow".
Now, I'm not saying it's not stupid that she got fired. When I was teaching I said some things that could have gotten me canned and that was almost 25 years ago. I didn't get fired then and she shouldn't have been released either, but let's also keep in mind she was a sub. It's not like a salaried, tenured person got fired here. A sub was told "not to come back". Little bit different.
What she could have/should have said is that cats aren't allowed in school and that he'd need to leave. Watch the office figure out how to deal with THAT. (They want butts in seats, it's how they get their money. But if he's a ******* CAT, he can't very well be in school, can he?) If they require him to be in school, technically they aren't respecting his identity. THEY'RE the ones who are bigoted and insincere.