Yes there is nothing stopping you doing whatever you want once you walk into that exam. There have been stories I have heard of people accidentally doing every option in the exam, effectively doing the whole paper. If you do this they just give you the mark of your best option and count that, a;though it would be kind of stupid. Don't do another option from the one you are doing at school, you just won't know it as well. Plus you will have to study 2 options because you'll be tested on your schools option internally during the year. Why bother?