I have to write a critical essay about the movie Freedom Writers. I would like to analyze three conflicts the teacher deals with. My ideas for the teacher's conflicts are (1) school staff (they are biased), (2) racism (or should I change it to inexperience with teaching?), and (3) the students don't believe her. Also, I'm wondering what kind of fact would be good to use in the introduction.

In a critical essay, the writer is tasked with interpreting and evaluating the text (or movie). This falls into the persuasive writing arena because the writer must make a claim and then back it up with facts—in this case, information from the movie Freedom Writers.
Since this movie is based on the nonfiction book The Freedom Writers Diary: How a Teacher and 150 Teens Used Writing to Change Themselves and the World Around Them, it is fine to pull facts from that as well, if you are using them to skillfully support your claim about the movie. For example, you could use the fact that all 150 Woodrow Wilson High School students in Erin Gruwell’s Freedom Writers group graduated, and some even went to college.
When presenting conflicts, it is usually best to start with a broad topic—for example, racism—and then use facts from the text or movie to support the topic. Lack of teaching experience could be a broad topic, as could bias. Instead of writing about the staff as the conflict, a broader approach would be to use bias as a topic, and then explain what kind of bias, and use the staff and other examples to support it.
http://www.freedomwritersfoundation.org/index.php/about-us/our-story

https://www.theatlantic.com/national/archive/2012/09/in-defense-of-the-freedom-writers/262800/