Thanks, everyone.
I cannot fault the school - they reacted immediately and sensitively, and have offered ongoing support to all concerned. DD has been offered sessions with a counsellor, and we are in regular contact with the SENCo.
Unfortunately, the boy is also in DD's youth theatre group, but he has absented himself since.
DD has said she would not consider our local uni, but we may revisit that possibility, and a gap year or even grades in hand is a strong likelihood. I just hope she can find a way to compartmentalize what happened, so that she can find her confidence again and enjoy her last year at school; despite everything, she is still very happy that she moved schools and loves being there.