I suffered a mTBI around the same age as your daughter, I am now two years older but still suffer from some lingering symptoms. All I can say is try to be there for her, I had no support and it made everything a lot worse. One of the large mistakes that I made as a teen suffering PCS was living in a bubble. I quickly began to suffer from anxiety and severe depression. I would recommend helping her try to find a balance between continuing normal activities, and providing a conductive healing environment for her brain.

Also you really need not worry about her GPA. I am a very high achieving student, and I was forced to take a full year off. I have had to repeat courses and take courses with younger students. I have now been accepted into many prestigious colleges/universities. She will be OK.

Regarding schoolwork I recommend trying to get extra time on tests and assignments, my teachers were very sympathetic and good about this. I had to take an extremely heavy courseload while suffering PCS, make sure she takes lot of breaks in between her work or she will tire out.