If your into sci fi, read alot of books, watch tv shows and such and know it well then you can do it. But if you just feel like writing one cause it sounds like fun, you probably wont do to well in an exam
You really have to be concise if you to attempt it because one thing you have to do is set up your world as being within the sci fi genre. And if its your first attempt at doing this you may spend far too long describing the flying cars and stuff and not enough time actually developing characters and plot meaning you end up with an average story