I really don't know what's permitted and what's not... I personally have 4 such blouses, 2 of them being full sleeves and 2 being 3/4th... and I wore only the 3/4th for my exams. The matric day was on the colder side, so I wore the full sleeves.
Looked at my matric photo now- nope, you needn't button right up to the top, and no one really cares how you tie the ribbon... you just need a ribbon there, that's all (unless of course, your college is very particular about it)