In my first year of sixth form I did this, too. It was a sociology exam and I had about 20% attendance and didn't know anything. I opened the exam paper and I thought "****, I can't answer this" so I spent the two hours or whatever we had just writing 8 pages or so to the examiner. That was cool. But after the exam I found out I didn't understand the questions because I forgot to turn the page, apparently the questions we were supposed to answer were on page 3 of the exam booklet.
I got a U for that exam, so you should be fine.