I have seen it and suspect the question depends on who your audience is. Some of the student's life situations are painful, and not necessarily what I would suggest showing to very young children. However, it was shown at our Hanna Boys Center to teens who had similar life experiences and was well received. Some of our staff have teenage kids and they watched it with no problems. When I watched it, it reminded me of kids I knew in high school, so I suspect high school-age kids might really identify with the film. The audiences for our showings were primarily adults who worked as teachers or social workers, and I do not recall anyone suggesting a viewing warning.
I recommend setting a few boxes of tissues out for the audience. My one criticism is that it is not a film that you can simply watch and say, "Well, that's what we should do." The teachers shown have some serious boundary issues; not inappropriate actions, but I would worry about rapid burnout. It's hard to imagine sustaining that level of effort for all the students in the school.