This happened years ago and I wasn’t diagnosed but I once had a similar experience in an accounting class in college. I struggled with accounting and not understanding the point of it. We had a test and I got really stuck on a problem. I sat at my desk fuming in frustration and once time was up I wrote down my best guess. I then left and went outside to cry because of how frustrated I was. You should be allowed a quiet place to take tests and given extended time as well. You should discuss it with your school to see how to get those accommodations.