While not really specific to hoodoo, I liked to draw little crosses for luck next to my name.
Also not restricted to hoodoo: you could pray for understanding, guidance, and success in things. I sacrifice to a local theos for aid before and after each semester.
Just remember, you still need to do hard work in school. You need to study, you need to listen, and need to do your assignments. Nothing will magically give you the knowledge to pass exams. Like everything, you have to work for it.