We've been there & then some with DD & I would love to type more, but it's late & I'm exhausted, so will need to be brief...
This link could be useful to you for further help & advice ...
HERE
& we had a very similar situation with DD - it won't get better, because the school aren't tackling the problem & sadly IME that just sets a precident that allows DCs prone to this type of behaviour to believe they are untouchable & it will only get worse - if not with this girl, with others as your DD will fall further & further into the victim role & her self esteem will be crushed & other DCs will target her too.
Mine was never quiet, always bubbly, bright, clever & very popular, she also loved to read & do her work well & throughout her time in her last primary school she was a target for this sort of behaviour - that was despite her also being very feisty & refusing to fall into line with the toxic friend who grew to be the ring leader bully - it's escalates & gets worse because the school don't donthere job & tackle it effectively.
I thought fighting & winning was a better life lesson for my DD than running away - we did fight & win - again & again with several different DCs - even this girl ended up being removed from the school by her DM who realised there genuinely was a problem & moved her to a much stricter religious school in the hope of sorting her out
But the girl that was her side kick took over & then there was others - boys looking for DDs attention & not getting it & getting nasty
I ended up with a DD so worryingly fragile that I removed her from school part way through year 6 & home schooled for a term - something had to give & I wasn't going to let it be DDs mental health
She's now in a new primary school to continue year 6 & what a difference this school is, no nonsense accepted with bullying at all - I now have my bright, bubbly popular girl back
My only regret - that I didn't do it years ago - it's not the toxic friend that's at fault - she is after all still a DC herself - it's the school who are letting both your DD & this girl down by not tacking the "bullying behaviour" efficiently & therefore not teaching those who behave in this way that you don't get away with it.
I echo those who say find a better school, your DDs current one are failing in there duty if care - move on
In the mean time, lots of out of school activities where she can make new friends to boost her self esteem - youth club for example
Good luck