Hi, OP -
We love our DC and want the best for them. Thinking positively rather than punitively, that should mean for the long term.
If DS is a healthy Y12 pupil, he is fully capable of making up this story. As PP have pointed out, phones lock very quickly and it isn’t easy to visualise what you’ve described within the classroom context.
I make no judgment on whether DS or a friend pulled this ridiculous stunt. I don’t see how you, who so want to believe the best of him, can do so either.
it’s very serious although from what you quote I would hope that it will not call for official action by the police, or compromise DS’ future. The message is arguably no worse than very crude and insulting, painting DS as an arse of the first order. If he really did let someone else do this, the consequences will hopefully be enough of a lesson that he will use his phone properly during school (not at all?) in the future.
But the only responsible thing is for DS to terminate the tutoring immediately. Even if he is innocent, the message to the Y9 girl could come back to bite him in all sorts of ways. If he is not, termination is imperative.
The message itself has contaminated the relationship irreparably. Maybe that is enough of a lesson to him. Learning it at this age is not the worst thing.