Great question! In this sentence, "it" is a dummy subject. This means it is not referring to anything, but simply anticipates the real subject of the sentence, which is the infinitive clause "for students to choose a good movie." This is a very common use of "it." Other dummy subjects include "there" and "here"— e.g. There is a bird in the tree. Here, "there" does not mean anything by itself. It is not referring mainly to a location. That fact is clearer in examples like "there is no valid reason to think the earth is flat." I hope that helps!