All I would say is that i have been through this with two of my four DC, and the answer that 'nothing is happening/wrong at school' is in my experience a compete lie, I'm sorry to say
. That is a pretty standard answer when they don't want to tell/can't put it into words/don't want to make matters worse etc etc etc. My DD spent two terms last year giving that answer, but being teary, staying up late and sleepless before school, mild 'illnesses' like headache, sore tummy etc to avoid school. School were contacted, they monitored, and reported back that everything was fine, 'quite the opposite', Dd seemed happy and in thick of it in class and so on. Eventually I took DD out for the afternoon, and put my arm around her somewhere and said 'I'm not daft, I know there's something up, I'm certain its at school, and if you don't feel able to tell me then I can't help you deal with it, and I'd like to, because I can see its making you sad'. Then I said NOTHING, just waited and waited for her to say something. Then it started to slide out, slowly at first. Four girls making her life hell, out of sight of everyone else, and she was too afraid of them to ask for help. The school had no idea. Once they were alerted, they monitored again, and caught the little darlings red handed.
It takes a long long time for some children to articulate the problem because they feel so oppressed by it. But I'm sorry, I would not put this down to hormones. Your instinct says she's unhappy, and I am sure you are right.