If you don't pass, you don't pass. There isn't much you can do about it.
Influencing someones choice encroaches on taking away someones free will, something that is both difficult and often considered negative. Surely if you can force someone to pass your exam paper, everyone could do it, who would need exams when people could use magick to force a pass grade.
Think of it this way, would you be happy if someone else passed the year if they didn't complete most of an exam? Do you know 100% that you would have passed the exam if you were in good health? How do you decide that you are more worthly of passing than anyone else in that exam room, even if they were also unwell?
These are just some things that should be considered before you even attempt a spell that could have a detramental impact on others, even if you won't get any result from the spell.