When the students come to the black light pep rally, keep it hushed until everyone arrives. Many substances and items that react with black lights, like posters, paint, jewelry and clothing, are either invisible or they just look white. If you get everyone in the gym for the pep rally and then you shut the regular lights off and turn the black lights on, the sudden change might be enough to shock some of the students into letting out a cry or two.
Glow Stick Gathering
If you want to both have a unique pep rally and make a little extra money for the school, you could sell black light items to students on their way into the rally. Glow-in-the-dark necklaces, bracelets, glow sticks and other accouterments might all serve to get the students into the swing of the rally. Add club or techno music to the mix and you're going to have quite the miniature rave going on, with everyone's attention directed toward the center stage.
If the game or competition is near Halloween, then the black light pep rally gives you a lot of opportunity to add some extra creep to the performance. Cheerleaders could paint up their faces like skulls so that they glow-in-the-dark. Football players who come in their field gear could smear red, glowing paint on helmets or hands to look like blood. If you add in some appropriately creepy music, then you have a full-on Halloween horror show theme going on.