A handhold from me, I am so sorry that you and your DD are going through this.
Please report to the police first and foremost, it falls under malicious communications and should be taken very seriously. Gather as much evidence as possible from your DD’s phone.
Update the school; tutor, HoY, pastoral, DSL and Headteacher. Hopefully your DD will share the names of those behind this. Arrange a meeting and seek reassurance on the measures that will be put in place at school (seating plans, safe spaces, mentoring, who to see for urgent support, the journey to/from school, protocols if your DD doesn’t turn up for a lesson etc)
I would remove your DD from SM to give her a break from the bullies. If changing schools is an option for you, definitely explore this. A fresh start can be beneficial, but SM also means that children have vast networks across multiple schools and word travels fast, be mindful of this.
Take all the support you are offered. Empower your DD, find ways to boost her self-esteem and confidence. Make her realise that life is worth living and she is so much better than them. Encourage her to take up a hobby to widen her friendship circle (if she’s sporty, girls football could be worth exploring, for example) and to give her a release.
Wishing your DD a speedy recovery and a bright and happy future 💐