I don't have ADHD (officially), though I do have some attentional issues (my last eval said I cannot do tasks requiring sustained attention, but for some reason, it did not use the term ADHD). Anyhow, due to a number of issues, I get 50% more time on multiple choice or objective exams. When it comes to exams requiring essays or calculations, I get 100% more time. I am supposed to get a distraction free environment, but the tutorial services are all closed at the college I attend currently (my undergraduate college had no such issues, but then I took classes during the day then).
I would talk to the disabilities department of your school. If you are studying and unable to finish your exams in the time and a half they give you, they should be willing to accomodate you. How did this work when you were in high school?
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