I am glad that we have a movie junkie here, but mind you, a film review is not an essay. It is not as boring as an essay nor is it as long or factual.
A film review is basically your professionally written opinion about a film. Since you are writing for school, you want to sound formal and well educated.
In reality, the film review format is not the same all around. If you are writing for a Goth horror movie audience, you can be as casual as you want. For academic purposes though, as is the case with you, the writing rules are a bit more formal. You should start by creating an outline of your ideas. List every thought or idea that you have about the movie. Also you should list the major conflicts from the beginning to the end as well as the main characters. You are the reviewer, so you are responsible for knowing everything that happens in the story. My best advice for you is to read as many as possible film reviews by renowned critics (e.g. http://www.nytimes.com/pages/movies/index.html)
and try to synthesize the patterns. You see, even though you cannot write whatever you like in a formal film review, there are still much room left for you to exploit.