After some advice from experienced parents of teens/pre-teens please...
Dd 11 in year 7 has come home in tears saying that a girl in her form who is also in lots of her lessons has been nasty to her today & previous day too.. Saying things like nobody likes you, your family don't like you & I'm going to tell everyone about your dead sister.
Dd said this girl is very popular & has already amassed a little gang of followers & turned a girl dd went to primary school with against her.
Now at what point do I intervene & how do I approach it?
I have told dd to try to ignore her & just walk away from her but she said it's not that easy as they have to sit together in some classes, she is also scared of getting labelled a grass.
So horrible to see her upset 
Thank you in advance.