I just want to say as a former bullying victim targeted by a former close friend, you are an AMAZING parent. Definitely do EVERYTHING you can to get this sorted now before it escalates because in my experience, it will. Ideally if your daughter has her phone on her can she try to record one of these over heard conversations as indisputable proof of the bullying? My school refused to do anything about my bully as it was "my word against hers" and they wouldn't jeopardise her education by suspending her or kicking her out in case I was the one lying!
It wrecked my mental health, i was suicidal walking in front of cars on my way to school every morning hoping to get hit so it would just end. There was a brief wave of relief when the end of day bell went but as soon as half way home i'd have rising panic of having to go back next day, be awake til 4/5am every night unable to sleep for fear and worry. my grades slipped massively and not one teacher took me aside to ask why, though i was desperate for one to notice and care. Sadly i kept it from my parents for more than a year til it all came spilling out one wednesday night as i just couldn't cope another day. My mum kept me off the thursday and friday and was straight in to the school next morning, told nothing they could do because i had no "evidence". But a senior teacher who walked in half way through the meeting pulled the bully out of class and told her about my mum going in and what was said! Needless to say she spread a rumour i had falsely accused her of bullying. She had been a good friend from primary from age 5/6, and in year 5 before she moved away to a different school (as her younger brother was being bullied there). She had actually been the one standing up for me protecting me from a male bully who would hurt me physically as well as verbal abuse in the middle of class. She had been so sweet and lovely and protective i was sad she left, and so happy when i got a phone call from her in the Christmas break of my 3rd year at highschool that she was transferring to my school. I was bullied by the popular kids and thought my life would get BETTER for her coming back. The first three days there she talked of nothing but herself and she was nothing of the girl i remembered. She was into drinking, smoking, partying, underage sex and doing drugs, as well as judgemental and nasty, totally the opposite of me and my friends, whom she got a straight "in" with as someone i knew. She turned on me on day 3 of being there, started off as "jokey" insults like you make occasionally with good friends who know you're just playing, but she meant it. It was a daily occurrence she'd call me something horrible as a "joke" then smirk knowing she'd hurt me. A week in i asked my so called best friend to back me up if i confronted her and asked her to stop, just stand by my side whilst i spoke to her and just say she agreed she was being mean and going too far, but she refused. I didn't have the confidence to stand up to her alone but i wish so much i had at that point. 3 months in whilst i was, as usual, last to finish dressing for PE she sent a rumour round the gym of all the girls in my year that i was a lesbian and in year 8 at primary i'd been consoling her over something that had upset her and tried to kiss her. Conveniently in her story it supposedly happened in front of her brother so she had a "witness". Everyone was giving me funny looks when i walked in and there were whispers and i asked what was going on and everyone said "nothing". I got a text that night of a girl i barely knew from my form class saying she'd said something horrible and would tell me next day at school. I was so shocked, but the worst part was my friends of 2.5 years, whom i had had girly chats with for years about which boys in school we fancied etc, believed her, whom they had known 3 months, over me. No matter how many times i told them it wasn't true and never happened i got the "It's ok if you're lesbian/bi, we won't judge or stop being friends with you, we accept you, just be honest" talk over and over. Her bullying increased to all day every day, even in lessons (sadly she sat either next to me or right behind me in every class) She'd be whispering constant insults, laughing at me, throwing things like rubbers and pens at me then tap me on the shoulder and ask for it back. She'd huddle with other girls and talk loudly about me so i could hear just like is happening with your daughter. I'd be told about their after school online chats about me, and the 2 people i thought were my friends stood by, watched me be torn apart and said and did absolutely nothing, they would even laugh along at her "jokes". I was off ill one day we had art. The week after, the day before my next art class a friend of her main side kick who didn't even like me took me aside to warn me that the class i missed was collages, and they'd cut a picture of a hippo out and stuck it in my art book and written all sorts of nasty comments round it, including "Nice photo of you!" and "Everybody here hates you" and this girl had been bragging about it and how she couldn't wait for me to find it next class. I was very grateful to her because i actually would have broken down in class of shock and upset if i hadn't been pre warned, so i took in a sharpie and scribbled it all out, but sitting down as books were passed out and seeing their faces sat opposite me not knowing i knew and waiting for the reaction was horrible. I was relieved for the 6 week holidays and had signed up to do some voluntary work with banardos, just to find out SHE was doing it too and in the same group with my other 2 previous "friends" so i couldn't get away from her even in the summer. We actually got on friendly during our time there, and i really thought things had resolved and changed, she even admitted to my friends about the kiss in primary being a total lie. I went back to school after the holiday thinking everything would be ok now, but it was like those few weeks never happened. She had her posse back, (my 2 friends weren't active in her bullying other than laughing along, which i now know was out of fear of being her next target, there were other girls actively joined in who weren't at the volunteering) In fact she was worse than ever. I was being threatened constantly with outside of school violence too, she hd a total of 7 brothers/step brothers, most in violent gangs, whom she threatened would target my home and my family if i dared ever speak out about her or stand up for myself. That was when i finally broke. It was constant, never ending, i had no friends in school, no friends out of school, i had OCD and by this point depression too, the added anxiety and fear i was a constant wreck, and i stopped coping with the OCD and it was spiralling out of control. My dad was so sad and devastated to find out what had been happening to me, every day half an hour after i got home from school he'd take a break at work to call me, ask how i was, how was my day etc, and every day through silent tears i'd lie and say everything was good. After my mum going in I spent that next final week before half term getting stared at, dirty looks and loads of whispering about me. She didn't actively call me names to my face or say a word at all to me that week which was unnerving, then final day of term walking out of our last class which was PE, i heard a popular girl ask her (prob not knowing i was right behind her and could hear) "So are you going to jump her now?" Seems the plan had been to physically attack me at the end of the final day of term and loads of kids had known about it. Thankfully she had a last minute thing pop up and needed to rush to catch a bus so it didn't happen but it really shook me. I've never been violent in my life and no doubt couldn't have brought myself to hit her or try to defend myself and she was more than happy to hit people.
That half term break finished and i never went back to school. I was medically ruled unfit to attend in person but my teachers refused to send any work home, they couldn't be bothered with the hassle of putting it together, but i now realise they have massively unfair workloads as it is so don't blame them anymore. I went 15 months with no school work what so ever, it got to 3 months before i would have taken my GCSE's when i finally got given home schooling by my local hospitals school. I got 45 mins of english, 45 of maths, 45 of history, and 90 mins of science a week, less than half what i would get in a real school, and was 13 months behind on classwork, coursework and homework. I worked my ass off day and night for those 3 months to take english lit, english language, science, and maths at GCSE, and 1 of the 2 history GCSE papers that counted as a half GCSE. I actually managed B's in maths and history and C's in my 2 english and my science papers, which was a miracle given how behind i was. I felt like a complete failure though. I was predicted 10 A/A* grades and was capable of them had that bullying not ruined my life. It was way too late for me though, My OCD had taken over with the loss of structure of a daily routine of needing to be at school. Once that routine was broken all hell broke loose and i became house bound, had to give up the idea of college, university or working and my life has never recovered. If i hadn't been bullied out of school my OCD may have never taken over how it did, i could have gone to college, then uni or a career and would actually have a life.
I do apologise for the rant, it was semi relevant and its something i've bottled for so long with no support.
I really hope it works out for your daughter, i truly believe part of her is putting on a brave face asking you to not get involved because she's scared to death of the gang up mentality of she dared "snitch" and that she'll be bullied worse by more people, but a persistent parent and the willingness to involve police and collect evidence of the bullying and go to any length to get it stopped is paramount to stop it ruining another life.