I'd offer my Attwood, but I've written all over it to point out his incorrect generalizations and conclusions. I'd take anything written in there with a grain of salt, maybe a shaker-full, as the generalizations are stereotypic and demeaning.
"Area of Special Interest" is a good spin on obsessive behavior. When an object or idea is interfering with improvement in behavior and social interaction, and is being used as an avoidance technique, and is being sought out as an escape from social interactions, then it is an obsession. There is a time and place for all activities, and having only one interest is detrimental to any social growth.
In 10 years, when I retire from my public school position, I'm going to be free of my confidentiality restraint and I'll be able to give some proper examples of how I've had success with my students dealing with obsessive behavior. I've found that they can have many interests and share them properly with other students without getting to the obsession level.
Setting limits on when a certain activity can occur is not punishment. Allowing varying levels of anxiety to occur, and learning how to cope with the anxiety is a good thing. Life is going to throw out a lot of anxious moments - kids can't be kept in a bubble forever. Learning to cope with anxiety in a controlled, safe, loving environment is much better than allowing avoidance behavior to continue.
I agree with the PP, who mentioned puberty, hormones, and all that other lovely teenage girl stuff. It could be that she's just not used to the changes happening in her body right now. Her pediatrician would be able to tell what phase she's in puberty-wise.
Direct consequences for direction actions has worked best for the students I work with. Setting ground rules really helps - "if you hit, you will have to stay in your room, on your bed, until we are sure you have calmed down and you are ready to apologize" would be my first step if it was my child.
It's not easy, I know. It has to be dealt with as an unacceptable behavior no matter the cause though, as you've stated she's getting stronger. What if she lashed out at a smaller child, or someone who didn't know her and struck back?