Is she a bully?

A few weeks ago, my daughter informed me of a girl on the bus that wasn't being respectful of her. This girl is in her class and is supposedly a friend of my daughters. She has called my daughter names and told her she hated her and didn't want her in her life.  She has even gone to the extent of hitting my daughter in the head with a back pack full of books. It happened again today. I am fully aware that 10 year old girls are going to fuss and fight one day and be best of friends the next only to fuss and fight again the day after. I was, after all, a 10 year old girl once. Kids can be mean and say hurtful things. I asked my daughter what her response to the other girl was and she informed me that she didn't say anything to her because she feared the girl would do it again, only harder.

My advice to my daughter the first time this occurred was to ignore the mean words. Even though my daughter insisted otherwise,  I had hoped that the back pack incident had been unintentional. But after it happening yet again, I'm beginning to be concerned.

I have had issues with the school before not taking me or my daughter seriously. They tend to brush any "trouble" under the rug as if it didn't happen. For example, when a teacher told my daughter that her hair looked stupid resulting in my daughters hair cut. I was unsure how to approach this but after considering my options I decided to send a note to the driver of the bus and see where it goes from there.

To make matters worse, my daughter is supposed to sing in the school talent show with this girl. Maybe they'll be "friends" again by then. 

Anyway, I guess what I'm asking is am I overreacting? Or is this little girl taking things too far this time? What would you do?