Kids tend to act differently at home than at school. Pay attention to how you react to her inappropriate behavior, and contrast that with what her teacher might do. (Which is not to say that you should act like her teacher; it wouldn't work anyhow, but this exercise might help you see how you can stem the brutishness).
In addition to all the detangler suggestions, how about changing the brush? When we got dd4 a "baby brush", that did the trick -- no more tears. (Though, yes, sometimes I had to still remind her that she needed her hair brushed or cut -- her choice. Hasn't been an issue lately, though.) Showing the cause-effect nature of regularly brushing (and using whatever other hair products) helped, too. Ie, the more you brush, the less tangles you'll have, the less of a hassle it will be.
When she feels like it, dd can brush her own hair, as long as she understands I will come behind her to finish up the job. And I don't bother with the whole ponytail fight. Every once in a while she'll request one -- otherwise, loose is fine. (Except for ballet class, when her hair HAS to be up -- but there's no fight there, because it's coming from the ballet teacher, not mommy.) I think she tends to looks like a wild child, but she gets A LOT of compliments on her hair.
