|The only rule, aside from the constant need to be school-appropriate (no profanity or gratuitous references to sex or drug or alcohol use), is that it be entertaining for the audience. How is that achieved? The same as with poems written for the page, yet different, too. As with poems written for the page, all the poetry elements are key: say something, be precise and clear and original, use imagery and metaphors and sound elements like rhythm and rhymes.
Two things might make a slam poem different from poems written for the page. The first is the performance component. It helps to consider a slam poem as not a poem read aloud, but a fusion of 50% poem, 50% dynamic stage performance. Therefore slam poems, always memorized, often use comical exaggerations, unconventional angles, surprise twists, and big emotions like love, heartbreak, or outrage. They also may have parts where there are voices to imitate or places to move around.
||The second thing that makes a slam poem different from poems written for the page is what pleases audiences. Because traditional slam rules limit poems to three minutes, slam poems tend to hover around that length -- which for many students writing them feels pretty long the first few times. They often tend to, though certainly do not need to, borrow a lot from hip-hop styles, using plenty of internal rhymes, rhythmic flows without a rigid rhyme scheme, and slang. They also tend to fall into one of three camps: the funny, the emotionally powerful, and the funny-and-emotionally-powerful. The third camp has the most winners in it.
A word on emotional power: the most powerful poems contain self-revelations and personal secrets and fears, rather than just outrage turned outward at, say, political leaders. If you can let your secrets and weaknesses and fears out and alleviate the tension with humor and not come across as self-pitying but as honest and wise-in-retrospect, you're a real slam poet.