while this makes sense assuming scores are evenly distributed, i certainly hope most people aren't getting <= 60% :)

Thank you for pointing it out.

Don't think it's a good practice to write some implementation in class declaration ( that is in a header file normally ).

I don't really understand why there is null values through the given array, it's like we need to handle exceptions that are supposed to be handled in the upper level where the given array is built.

Nice idea I very like it.
That could works if a, b, c will be random in the real life we could check what what grade is most common.
But for the sake of readability, I would prefer either ascending or descending order.

Wouldn't it be a good thing to handle the 'F' case in first as it has 60% of chance to come in order to increase performance of code ?