There are quite a few errors in the program. I am mentioning the one I just saw when I looked at it.
1. scanf for string should be %s instead of %c.
2. score function you are using the variable i which is never initialized.
3. You are taking the input in score1 the scores but comparing with score1[i] where i is never initilaized.
4. Same is the case in score1
5. similar things are observed in grade function as wel.
I think the above points will help you rectify the problem you are facing.