According to the author Charlie Taylor, who wrote Divas and Doorslammers, during adolescence the brain actually rewires itself - very real changes in the bran (or so I understand), that cause the child to lose some abilities - empathy, sense of proportion, impulse and temper control and some common sense. He describes it as almost a form of temporary brain damage - and I do think it explains a lot of teenagers' more challenging behaviours.
I bought a copy of his book when we were struggling with ds3's behaviour when he was a teenager - he had a hair-trigger temper, and no ability whatsoever to see the effects of his behaviour on himself or anyone else. I was at the end of my tether.
I did read some of the book (though not all of it) and ds3 and I both managed to survive his teenage years intact - and he's now a hard working, kind young man who is much better able to control his temper and be sensible.
I am saying all of this because, whilst the specific problem that you are having with your ds, @Sewbean, is different to the problems I was having with ds3, I suspect they both have some common roots, in this brain rewiring thing. And I know it doesn't necessarily help you to cope with this particular incident, but knowing why something is happening does help you cope with the feelings of frustration.
If I were you, when he gets home, I'd sit him down and say something like this - "I want us to get out of this deadlock we are in. You can have your phone back (but the normal rules about handing it over at bedtime will apply), and I'd like us to have a chat about your French class and the teacher, and find out a bit more clearly what the problems are, so we can decide how best to deal with this. Are you willing to accept your phone curfew and to have a chat with me?" And hopefully he will have calmed down enough to say yes, and you can take it from there.
Good luck - raising teenagers is a challenge!