M.C.
Do you pick them up after school?
Go to the store and buy two special treats - dessert, candy, latte, etc. Have them be identical. Put them in two bags. Bag 'Yay, for A' and Bag 'Not and A'.
When you see her, ask her what grade she got. If she got an 'A', give her bag 'Yay for A'. If she got something else, give her 'Not A'. You keep the other one. Open them at the same time.
Good luck.
M.