I think a speech on cliques would be more attention grabbing. Especially during secondary school (elementary, middle, or high school), cliques are something that everyone has experience with. Of course if you are simply identifying what's already obvious, you'll more than likely put everyone to sleep.
Here's how I'd go about hooking my audience in a speech about cliques:
I would begin with an anecdote or attention grabber. Some story about some fictitious (or real, if you can think of a good example) member of such a group that did something attention grabbing within his or her selective group. Perhaps they had to do something to prove themselves, or perhaps they dared defy the "party line" by associating with someone outside of the group. Then once I have the audience's attention with this interesting side story, I would go into the point of my persuasive speech.
Aim for sharing something uncommon. Humor as part of your opener is always a good attention grabber. And conclude your speech with some kind of lesson that ties everyone together--some kind of universal human truth that we often forget when we're so busy hanging around people identical to us.