Perhaps the most important step you can take is to try and help your child deal with the reason(s) she wants to quit.  Usually there are issues of poor attendance, lack of motivation in class, poor performance in the classroom or other personal/social issues.  Getting to the core of the problem is so critical.

Many students drop out of school towards the end of the year because they are failing many subjects or have missed so much school that they cannot earn credit because of attendance.

Bottom line- it is fairly difficult to 'force' a 17-yr old to stay in school- they often skip school without your knowledge, go to some classes and miss the rest, and a host of other behaviors.

Get to the bottom of her issues and try to work with school officials to help your daughter stay in school.

Good luck!