Ally = my source for science homework questions. and this year i'm doing chemistry, and i'm an embarrassment at chemistry.
Duelling with the elements would be way cool, though. you could include light and dark as elements. and you could have telekinesy, so they could lift things with their minds, or throw pure force!
hmm... if your MC is captain of the club, you could do something cool, like, the old captain got kidnapped, and she ends up replacement but everyone hates her. and then she could be a total BAMF.
ETA: Ally, if you like that idea you should totally read my BFF's books when they come out. It's about a world where there are special children, currently called angels, who are born with supernatural strength, speed, and an extra power. There are two special schools for these children: but one is recruiting them for an army and another is dangerous experiments that kill half of them. And then there's a third race of children who pop up, with computer chips implanted in their brains with specific codes that could control or kill them without warning. Only problem is, no one knows who implanted the chips...