Mother of a DS who went to a famous art and design school for Graphic and Digital Design. Here is my take.
I am not sure that her being a HS "slacker" would be my concern. She has chosen a higher education path that is not the traditional academic path. That's fine.
If I were you and this how we kept DS motivated, I would emphasize that the need for good work habits, study skills and diligence will be absolutely necessary in art school. Art school is VERY demanding and depending on the field, ultra competitive. A lot of students do not understand that. You cannot just show up and do well. So I would stress to her that NOW is the time to develop the high standards for herself that will be an absolute must at the next level. Pushing herelf now with no relative pressure will help her later. So I would focus less on the GPA and more on the development aspect of it. GL