You should let them know that either way you are their friend and don't try to lead them to what you know is right. Just being their friend always lets them know you're there for them. Also, you can advise them to seek counseling or talk to an adult about why they are not wanting to continue going to school but don't be too pushy.
You may consider asking them why they feel this way. Could it have to do with the educational curriculum? (As a woman of color, I do not like how Eurocentric our classes are and how the curriculum does not highlight enough contributions by women and people of color.) I would also be concerned if a friend became disinterested in school due to bullying or problems with family members. Whatever the case may be, it is vital to form a strong support group for each other and work towards maintaining a healthy and inspiring learning environment.