Bulling

So I'm still in school and there are these girls that are bulling my friend, Emily. She corrected this girl Cassidy because she said 'In Saturday' instead of 'on Saturday' and Cassidy cussed her out (she has issues) then all these 'popular' girls were asking me who Emily was too. I was walking to class with Emily and one of the popular girls Monica yanked Emily's ponytail really hard. And in Gym class another girl, Cora was mocking everything that Emily said in a high pitch voice. Emily doesn't want to do anything about it and ignores them, but I can't stand to just watch. I don't know if I should say something, go to the office or leave it? It's only been four days, and I think they have done some other things that I don't know about. What should I do?