furious this is awful.
When you say "...so all the name calling and rumour spreading is now coming at her via other pupils who would be very unwilling to co-operate."
Do you mean these kids are at her school and are telling her what is said to kind of let her know, or they are continuing the bullying? Because if it is the latter then the school can discipline them because they are part of her school.
The fact these are adult men picking on a child, a girl, is very worrying and I would, to be honest, consult the police. What interest have these men in bullying a child. Is there any evidence of this on her phone, or any screen shots etc?
I'd tell my dh, and make him promise to be calm but I would tell him and together decide how to deal with it.
The school can and should deal with this. Ask for their anti-bullying policy (or find it on the internet) and ask for a meeting where you go through this with them. If they say they cannot do anything do they want the other parents to know they are 'powerless'.
Check out these documents, google for similar, and be prepared to know what you can do, together.
www.gov.uk/government/publications/preventing-and-tackling-bullying
These are adult men, if it becomes known they are bullying a child they would hopefully be a laughing stock.
You need to get your dh on board but make him see losing his head could lead to these fuckheads winning, if he gets arrested for threatening behaviour.
Talk to dd, build her confidence, ask her how she wants this dealt with, can she just avoid these girls, can you get her some assertiveness training. Make her a part of finding the solution so that she feels empowered.
Sorry this is happening. The men and the girls helping them are utter shit.