I'll say what I said last time this sort of question was asked:
hmm... sci fi... different topic for me but we'll see
Mostly, robots, aliens, zombies, space adventures etc are used for sci fi and so are kinda overrated. You might wanna steer away from them but it's up to you. Just remember teachers like uniqueness
Really, it depends how you write. If you're good at romance, use that. If you're good at hurt stories, write one. Use your stylings to your advantage.
Personally, I'd find it easier to write a story containing sections of sadness, romance and thriller. So maybe something along the lines of a timetravel where people meet and fall in love or an apocalypse of some sort where there is but one survivor. I dunno.
You can also always adapt something from a scifi movie/game/book etc. Like Resident Evil. If you haven't played it, in there Steve is turned into a monster and Claire and he are in love. So you could make something similar, or a sort of thing where you're extending something, like a lonely extenstion of E.T. Just sayin.
Wikipedia has an extensive source of subgenres of Sci Fi which could also help
That's all I got I'm afraid, hope it helps and have fun writing and letting your imagination loose (my Yr 11 Eng teach won't let me, she's got us doing analysis... -__-) and good luck with your grade