Ah, I've always had exactly the same problem- for English I end up writing literally half the essay in a 45 minute test.. I'm talking one and a half pages at times haha , VERY embarrassing but I still somehow manage to get 75 % for that. The teacher told me that the only reason for this is that i'm trying to write a piece of professional writing in that time and that's obviously not the point of a test..
I only do this in English actually, and I try not to focus on the standard of wording and whatever, but usually I just can't escape it! It was better for the half yearly but i'm rather worried that I'll stuff up the final HSC exam because of this insanity, when my practise essays that I hand in invariably get 100%. It suckkss.