A.V.
my first thought is that there is something going on in her life that is causing her stress or pain. If she won't talk to you call her school counselor and tell him/her what is going on, they can then call your daughter in and hopefully be able to get some insight into what is going on. The years 11 and 12 were very hard for me, I was very shy and awkward and got teased at school. My parents were completely oblivious to it as they were having there own problems at the time. My school counselor helped me a great deal, it doesn't hurt to make a phone call and sometimes another perspective can help a ton.
Good luck.