An 8th grader may not have control over how their time is spent, but an attuned response from the people around them will help the child adapt.
A child experiencing: 'everyone around me can take this test, but i cannot, I must be dumb'
vs
A child experiencing: 'everyone around me can take this test, but i cannot, I must be dumb' and a caring figure in their life explaining to them 'you show traits of ADHD, this commonly makes it harder for you to focus on things like a math test. it really hurts when you fail the test and you wanted to get an A. Why don't you try again at the same problem at home, I believe in you. And maybe I will talk to your teacher about some extra time for the next test. We can't always get this, and even if you don't pass the test it's ok.'