Is she introverted/shy? I was like this when I was a kid, and I lied because I didn't like the attention that came with owning up. I didn't like lying but I didn't want to say I did something wrong and then have to have a whole conversation about it. Especially if asked in front of other people if I did something, I would always lie. My parents didn't handle it very well, so I don't have any suggestions there but could you change your approach or talk with her more about why she doesn't want to get in trouble?