The law of Texas is controlled by federal law, viz. the First Amendment. This item by the ACLU summarizes what that law is. Regarding your concern, public schools are not required to be "religion-free zones". Individuals have the right to hold and express their religious beliefs, or their lack of such beliefs, as long as it is contextually appropriate. That is, you don't get to invoke the First Amendment in the middle of a math lesson in order to engage in a rant. Students are allowed to try to persuade others to believe whatever they want, and students are allowed to be snarky about people who don't share their beliefs. Schools can teach about religion as part of the curriculum, though they cannot teach religion. It is okay to teach the historical fact that some guy decided based on the Bible that the world was created 10,000 years ago, but it not not okay to teach as a scientific fact that the world was created 10,000 years ago.
Although I can't name a specific movie about Christmas that would be plainly objectionable to a non-Christian, we could assume that there is some such movie which is plainly advocacy of a religious viewpoint and which mocks Yazidism. A Yazidi student could then be excused from this movie, just as Christian students could be excused from attending an analogous Yazidi-advocating movie.