I at least can certainly see why it won the Palme d'Or. The whole cinematic language of the film was a breath of fresh air and what could've been way too arty and boring and distant turned out to be a very poetic lesson of film making.
I don't know how well you know high school life in the United States (have you ever been in one?), but I don't get your cliche comment at all. They were sort of normal (not cliche) kids who's life was treated in this distant, detached, rather bizarre style. I don't see how you can use the term "cliche" in anything about this film. And using this style to sort of recreate Columbine was nothing short of genius, in my opinion. But yeah, you have to be patient and well into art house cinema to enjoy this.
Van Zandt has done a few movies in this style, and I think it's by far his most creative, artistic fair. Don't mean to be so contrarian to your review, but I thought this movie was a work of genius, and much more interesting than any real life dramatization of this tragic event could've ever been.