I had similar experiences as a first year teacher in a school like that. I think that all people want to belong, but as a defense mechanism we develop walls to keep others out. That way we are safe from rejection. Teenagers, especially this population, are masters at these walls. It is the teacher's job to get through.
You could look at the negative elements of "belonging" in this movie by the student involvement in gangs and Mrs. Gruwell's attempt to break some of the lines of hatred and discrimination in her classroom.
I love the scene where she re-seats all of the students and they are so obviously annoyed and angry (many of them physically put off) at their new spots. It is a great visual of the individual desire to belong somewhere and at the same time disgust at not wanting to belong in certain groups.
In my mind, I think that the most immediate way that the film connects to belonging lies in Gruwell's attempt to bring a sense of community and solidarity in her students, whom society deems as outsiders. The themes of marginalization are evident from the intial developments of the plot. The fact that Gruwell's students are referred to as the "unteachables" helps to bring to light that these individuals are the results of social and institutional marginalization. Gruwell uses this as a means to enhance the idea of belonging and the consciousness of solidarity. The belonging idea is further enhanced when the racially heterogeneous classroom is racially segregates pockets. Through Gruwell's teaching, belonging is something that she integrates into her approach to educating her students, forcing them to accept that they can create a realm where all of them belong, even if society does not reaffirm this.