I agree with the top voted answers in that there is clearly some string and compelling reason why she really doesn't want to go to school.
Consider that she in in a situation where everything in here life is forcing her to do something that she really really doesn't want to do. This is something that it is not necessarily easy for an adult (living in a modern democracy) to comprehend fully as while you of course have all sorts of pressures you are rarely actually forced to do anything against your will.
Also understand that it may be extremely difficult for her to explain what the problem is. This doesn't reflect on you nor doe sit man that she is being difficult or secretive it is probably just very hard for her to articulate.
I would suggest that the first thing you should do is to tell her that you realise that she is having problems and tell her that you are unconditionally on her side. Bear in mind that she may well be aware of expectations you have on her (even if they are in her best interests) and hr problems may be compounded by a sense of disappointing you.
On a practical level one of the best things you can do immediately is to give her some options. Even the sense of having a choice can make a huge difference in this sort of situation.
Don't expect to find out the root of the problem immediately or even ever but what you can do is give her a sense that she is safe and secure at home and doesn't need to resort to deception is she is scared to go to school.
Personally I would say that if your child really doesn't want to go to school they will benefit more from having your backing in solving whatever problem there is than just blindly forcing them to attend.