I've just done 3 AS subjects and a full A level this year, and my GCSEs last year. I found that even though I didn't feel stressed out last year, the subconscious stress and never having a chance to relax meant my blood ran very high, sometimes above 20 through the exams. Also not exercising as much as usual probably affected this.thankfully the grades came better than I ever expected, although if I'm honest I could have maybe improved 3 of my grades which were really close with better control.
unfortunately i didn't really sort my BGs out this year either, and because of that i really struggled to revise. In my mocks I actually had a hypo, but during the main exams my blood was often high again.
Sorry I know this post isn't particularly helpful, but I figured you might like to know your not the only one t0 struggle with diabetes and exams.