I may be a bit late with a suggestion/opinion (your results might be in!) but the only way to accomplish a lofty goal that breaks outside of linear time (a test you've already taken) is to truly deploy the Deep Mind in your work. Here's what I mean. Linear time in my opinion is an illusion (but that's partially because I'm into Chaos Theory (the mathematical equations as well as the magickal non-system), but also because I've been a Tantric Buddhist most of my later life. But the problem is, overcoming the conscious mind.
Now that the conscious mind believes it's TOO LATE, you have to empower the subconscious mind with the ammunition it will need to overcome this nearly impossible barrier to success (liner time equation). But, it can be done. For example, instead of trying to exert your WILL over the inanimate (that is, the test), why not create a sigil or servitor designed specifically to influence the person grading the exam (influencing them to giving you generous marks). Path of least resistance. Then, the subconscious simply doesn't have to deal with temporal doubt, and can instead connect via the Deep Mind (the theory of all minds are connected through the subconscious to the Deep Mind of all sentients). Perhaps you go for a "smaller goal" such as "sympathy" from the teacher, enough to allow that teacher to give you a chance to retake the exam. Then you try to influence the present when you retake the exam. There are many creative ways to approach this, but there's one thing I know for certain:
IF YOU WANT TO BE SURE... connect to your subconscious first with your preferred divination. The tarot, I Ching, whatever gives you perfect results in divination. Divine your different options. IF I TRY TO INFLUENCE THE GRADING OF MY EXISTING EXAM... VERSUS IF I TRY TO INFLUENCE THE GRADING TEACHER, VERSUS... etc. Let your subconscious tell you what is possible. If your subconscious tells you that you have little likelihood of influencing the outcome on your existing exam (either because of conscious doubt, or a willful teacher who can't be influenced, or just your mood this day) then you can formulate another creative option. Like having the sprinkler system in the school come on and destroy all the tests (JOKING!).
A divination is your best adviser in this complicated area.
I forgot to mention one thing. A friend of mine told me a story of an exam he took more than twenty years ago. He knew he did badly. That night he dreamed that he walked into the teachers office and changed all the answers. Now, maybe he sleepwalked (unlikely with security) but more likely his subconscious mind did something. Well, the teacher called him into his office to give him a lecture on cheating, because he ended up getting a ninety. The teacher couldn't believe the mark. But she couldn't prove otherwise, so he kept his mark. To this day, he swears he did nothing but dream of changing the answers. It's along the lines of the same principal of sigil magick, because it was, in fact, his subconcious mind that willed the change. True story.