Perhaps the parents of the other children have shared with her that they find the content inappropriate for their kids with the movie coming out and it coming more into the awareness of other parents. I'd have to expect the teacher to comply with other parents' requests of that sort if that is the case regardless of whether they make sense to others (i.e. - no greater violence than the video games they play).
If she were telling your son that he couldn't read it due to other kids' or their parents' objections, then I'd have more of a problem with it. Sorry to play devil's advocate here
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