Thank you for your question and welcome to ADHDCentral.com. Many teens reach an age when they don't want to take medications, it is so important for teens to fit in with their peers and taking medication and having "a disorder" makes them feel different and to them, therefore, "wrong."

In addition, since your granddaughter has been struggling in school, she is probably feeling discouraged and doesn't see a reason to try. Self-esteem can be a big issue for kids with ADHD.

I also believe that if treatment is not working, it may be time to talk with the doctor and the school and have a complete evaluation done (physical, emotional and educational) to make sure you are working with an accurate diagnosis.

Hopefully, your daughter is working with the school to come up with some accommodations or modifications to help your granddaughter succeed.

I know this is a difficult time for all of you. Hang in there, your granddaughter will appreciate how much you care for her, even if she is not always willing to show it.

I am providing you with some links to additional information that might be useful. 

Self Direction and the Transition to Adulthood

Failing, in Order to Succeed

Teens and Peer Pressure

When Teens With ADHD Are Defiant

The Fine Art of Raising a Teen With ADHD

When Treatment Isn't Working - The Importance of an Accurate Diagnosis

ADHD - An Accurate Diagnosis is Just the Beginning

I hope this information helps. Please let me know how it is going.

Eileen