If you are the guardian and caretaker of your granddaughter, then you have every right to know how she is doing in school. You should get your information from the school, so I would suggest a meeting with her school counselor, her teachers, and you. Let the school staff know you want to learn what you can about why your granddaughter is having difficulties. What are her abilities? What are her weaknesses? How can the school help? Work out a plan together. Make sure to include your granddaughter's ideas and opinions as much as possible. Your granddaughter may rebel at this, but you can firmly and calmly assure her that this is what you have to do and that it is your responsibility.
If you are not her guardian, then I would suggest you simply assure her of your love and support, and step back and let her parents work with her.