Is it fare to ground a 15 year ol girl who has WaterOnTheBrain,ADHD and depressi!

Is it fare to ground a 15 year ol girl who has WaterOnTheBrain,ADHD and depression issues because of her grade

Yes - and here is why. Parents set a certain standard for the child, if the request is unreasonable (like 100% straight As, and AP classes) then the child needs to talk with a teacher who plans IEPs -(Individual Educational Programs) and then have a meeting with the parents, student and educator to discuss the work load. However, if the child knew about the parents expectations before and did not have a reasonable adult, heart to heart talk (ne yelling, no "life is unfair" stuff) then by not saying anything reasonable, the child agreed to the terms the parents set.
When you begin to put people in special categories because of something they can or cannot do, then the rule of law becomes negotiable. IE: Look at Paris Hilton - special treatment in prison because she is a celebrity. Any other person would have gone straight to jail and none of the legal crap would have happenend. She broke a law she needs to pay the penalty.
Now - in order to avoid this situation in the future, I suggest that you sit down your student and parents and a educator. Plan some more reasonable goals. For now, grin and bear it, accept your dues and remember that disease is never an excuse for failure.