Definitely bullying, known as "social bullying"
Contact the HT via email only. Create a paper trail as this is very serious & the school are failing in their legal duty of care to your DD.
Who was it you spoke to ?
I once made the mistake of not realising that a more chatty/forward TA wasn't a teacher & once got a dreadful dismissive reaction from her when discussing a boy forcibly kissing DD, "only a game of kiss chase apparently"😏
My DD went through years of this, though ironically, in her case it was because she WOULDNT join in bitching about others & said so, though her toxic friends/bullies said she was the one bitching to teachers 😏 ringleader "frenemy" was incredibly manipulative & quite shockingly so given they were younger when this started
Make sure your DD gets support too, counselling if possible, this had an awful affect in my DD for years. Though over the years all but the ring leader have apologised to DD & the RL outed herself as obsessed with & wanting to be DD & has tried to show off that (apparently now Uber cool) DD is her old friend as it gives her cool brownie points ... DD took great pleasure in telling the people concerned that she doesn't really remember the RL
School MUST act, that is their legal responsibility to your DD & you. Make sure they interview the girls separately too, one at least will own up to what's really going in. Interviewed together they will back each other up & be too scared of repercussions to speak out
You can also go to the police as the online stuff at least warrants that & they are old enough
I also found the EWO a very useful person to contact too, they are there for the kids welfare, not the schools
Good luck & if you still don't get any joy with the school, consider moving schools as that is my biggest regret with DD.
We fought the school over bullying, won time & again to push them to act, I stupidly thought fighting & winning as a beer life lesson for DD & DD didn't want to move school, but reality was it was a shit school for dealing with bullying & as such bullying was rife in school & the best thing I ever did was move her to a better school at the end of year 6. She had 6 months of remembering how much fun junior school could be & that all girls weren't awful & there was nothing wrong with DD, but her ex friend. I will always regret not doing it sooner. I did use the bullying to get her into our next nearest very popular & oversubscribed school via an email rant about how badly the school had supported DD etc to the director of education.
It is absolutely bullying. Take no shit, invoke your inner tiger mum
Good luck