Puberty is a big thing to deal with, my daughter started around the same age as yours did. It sounds to me she's dealing with conflict between you & dad and that's really too bad. If her dad is expecting perfection then he's expecting too much. ON the other hand, if he's saying "school isn't important" thats a big issue as well. Your daughter may be trying to please everyone & that is her problem. All you can do there is keep reassuring her that while lying is wrong, telling the truth won't get you into as much trouble.
I would continue doing what you're doing. Talk to her, keep an open line of communication, let her know she can talk to you about ANYTHING and if she needs to confess, assure her you won't punish her for telling you, but she has to expect consequences for her actions. Let her know that you don't expect perfection from her, only her best, and if she shows that she's applying herself that's reward enough.
I didn't put much pressure on my daughter, I did get lucky in that she's not like most teens (ok she's not like I was as a teen) but she's working to keep a 3.5 average in high school and gets upset when her grades fall close to C's. I tell her to do her best, and if she needs help I'll find it for her. She's not lying, but the pressure is there none-the less. Gotta laugh tho, the other nite I asked her what she was doing at this party she wanted to go to and she said "drugs & animal sacrifices" I can laugh about that knowing my daughter, she tries to be dramatic for me, but it doesn't work. She didn't come home stoned or covered in blood either!
Off the subject a bit, Adelmaal sorry! Show her you support her as much as you can & chances are this is just a phase.
SK