Your daughter is 自慰. WHy I know,

I did the same thing when I was her age. It made me feel really good. My parents always told me to
stop it. I felt that I did something very bad.
But I still did it when my parents was not watching.
You cannot watch her in the school.

As a mother, I know what you worry is. No matter what you do, do not make her feel that she is doing bad things. You would better consult her doctor.