I personally really wouldn't be calling for anyone's heads without having witnessed the tone of the exchanges with the teacher, and future discussions. All kids have a great ability to explain to parents that they were 'just' doing one thing, while leaving out surrounding details that change the tone of a story immensely.
Using French phrases back and forth with one another during an English speaking class would have gotten me into just as much trouble in school with my teachers, especially if we were making a show (which I know, teenage girls never do, right?), of having our own exchange going on behind a teacher's back.
This is the way I feel too. I grew up in an area with a large Cree population and some of the kids (white and aboriginal both)would use Cree insults/profanity, with the thought that they would get away with it because they wouldn't be understood, especially when dealing with new teachers, fresh out of university. Its the double-edged part of dealing with kids in multi-lingual situations. Now the girl and her family claim her actions were all innocent but how is the teacher supposed to know what is being said? I'm willing to bet teachers fluent in the native dialect aren't so common as to make it a reasonable job requirement for that area...