I would like to share my daughter's story, a student who visits the grave of The Tzadik of Zvhill before every semester, where she donates to the Rebbe's holy institutions, and prays for good grades, C+ at least. To this day, all of her exams were graded above C+, just like she's asking for.
During the last semester, one of her exams was graded C-, which highly troubled her. But in the very same day she told me: "I prayed at The holy Rebbe and he promised to be there for me. I'm sure he will help me. He has yet to fail me."
To be honest, I was troubled by the fact my daughter was about to appeal her low grade, but she was determined that if the Rebbe promised to help her – she will get exactly what she asked him for.
I was no match for her confidence, and she did file an appeal. To my amazement it was approved, and her exam grade was changed to B.
That day I saw how powerful indeed is the belief in God and in Tzadikim.