I mean as I understand it hyperlexia isn't really considered a medical condition so its hard to say what the diagnostic criteria should be. I'd say being 5 years ahead for your reading age would have to be a statistically significant gap though. The thing about school is they mostly test your ability to communicate what you've learned not what you've actually learned. If you're bad at expressing yourself in writing you'll do badly on most exams, except maybe maths where, as long as you show your working, you should be ok if you actually understand what's going on.
A love of reading doesn't actually mean you're automatically good at forming easy to follow sentences. I often think oral exams might be better for students doing essay style questions because the examiner can stop the student and tell them that the meaning of their answer isn't clear.