You know how you're suppose to study hard and do all your work and then you can get the grades you want? It's not exactly working for me.
I've completed all my homework, did extra studying, and when teachers give me assignments, I complete them without a problem- hence I understand everything. I even help other fellow classmates and friends. But when it comes to an examination like a test or quiz, I mentally believe I'm doing it right but when I get it back, it turns out I screwed up in the stupidest way you can imagine. This has been going on for several of my courses, and since I'm in Grade 12, I can't afford to do anymore mess ups or else the marks are gonna affect my future university paths.
Help. What can I do?

That's really bizarre considering the amount of work you seem to put into it. Well, I don't study much, I just do a lot of work and my brain does the rest to memorize as much as it can in the five minutes I do decide to study for. I guess it's whether or not your brain is capable of retaining information. =/ I'm guessing you make connections in your notes, but if not you should do that and study those to help you out more. Also, use the elimination process for multiple choice tests/quizzes. Dwindle it down to two possible answers (except during English class, in which case you may be totally screwed). Also, making songs for what you're learning helps a lot too! Rewriting your notes (in blue pen if possible) is even better. Listen to classical music. All of this helps. =) Hopefully it works out, because you wouldn't want it to impact your transcript. Good luck!

Since you do better at regular assignments and homework, try treating a test like your assignments. This might sound bad, considering each of your tests count, but for your situations, it might work. Reverse psychology I guess kind Of?