Specifically in schools, this is problematic because it makes children who do not celebrate christmas feel left out. Adults should be able to handle it, children not so much.
> You can't say you understand homosexuality to be a disorder.
This is not an opinion related to your job, so you should definitely keep it to yourself while on the job. While some people will definitely throw a fit about it if you make the opinion known outside of your job(especially regarding teachers) I agree that this is bad and shouldn't be done.
> You can't call a man "he" if he identifies as a "she".
This is just simple respect. If someone asks you to call them something, you call them what they asked. Even if you think it is wrong/ridiculous. It doesn't hurt you any.