I imagine it must be so very frustrating to see your grand-daughter struggle with school with little direction as to how you can help her.
Unfortunately, it is very difficult to answer your question without additional information about her traumatic brain injury (TMI) or the extent of her disability. Do her parents feel like they have an understanding of their daughter's condition, or are they also uncertain about what she is capable of? If they understand your grand-daughter's condition, I invite you to sit down with them to learn more about her condition and strategize how you might be able to help your grand-daughter.
If her parents do not have a strong understanding of their daughter's condition, you might talk with them about setting a meeting with her school to discuss her current progress. Ask to be invited, so that you can hear from your grand-daughter's teachers how she is doing and what can be expected of her.
Finally, if her school is not helpful in terms of elucidating your grand-daughter's condition, you may consider reaching out to a neuropsychologist (a psychologist specializing in brain functioning) to get a more thorough assessment and plan for helping your grand-daughter. Ask your grand-daughter's school for a recommendation, look in the phone book, or conduct a search online for your local community and child neuropsychologists.