Most of the reviewers here speak from their own viewpoints, i.e. non-Japanese westerners, and they praise/knock the movie based on its violence, plot, etc. That's fine. But through their ignorance of the culture this film springs from, they are missing its subtleties.
I've been teaching in a Japanese high school for three years now. Once I saw this movie, I could instantly appreciate its skill and surprising frankness at commenting on some of the sad and strange realities of Japan's modern youth.
Japan is a culture obsessed with youth. Almost everything here is tailored to the under-30 (and much younger, actually) crowd. For example, most westerners watching Japanese TV will be surprised at how childish it seems. The things that seem childish to your average American junior-high student are very appealing for a Japanese high-school student. Girls in their 30s desperately try to be "cute" to attract guys. Adults and children alike read comics by the droves, and sometimes pops up a strange, not-too-well-hidden undercurrent of pedophilia.
This movie takes the heavily cliquish, often childish, and often incomprehensible (to me) social system of young Japanese boys and girls and gives them guns. This is the natural result. Take it from me, the characters and situations are very realistic.
This gets mixed with the growing anxiety among the older generation at the rising rudeness and rebellion of the new generation in a culture that values politeness above all else. From a frustrated and humiliated teacher; to students killing each other over seemingly unimportant squabbles; to the overly-cutesy, peppy training video that perfectly mimics nearly any show on NHK these days -- this film subtly and brilliantly comments on half-a-dozen issues that weigh heavily on the minds of Japanese people today. That's why it was such a big hit in Japan.
Maybe you just have to live here to get it. I give it 5 stars.