Brad's always been ambitious.
Which means he's learned to be smart where it counts. Not geek smart, but smart enough to hide how smart he is, and how to make the cool kids tolerate him. A little flattery here, some clowning around there, the occasional thoughtful gift. Before you know it, you're not a loser any more. A brown noser maybe, but not a loser.
Hey, you claw your way up that ladder any way you can, right?
So when he finds himself in this stupid sitch - sired by the dumbest girl in school, rubbing shoulders with giant assholes like Mort and losers like Cyrus and Peaches - and what kind of name is Peaches for a guy anyway? - he resorts to what he knows best.
Okay, so Harmony may be dumb, but she's the leader of this gang (for now), and the one with the lair and the plan. That means keeping on her good side, and that means presents.
The china unicorn is hideous. Brad's mom (tasted like chicken) wouldn't even have it in the house. But Brad remembers from the time he stood Harmony up in tenth grade- in fact, that's why he stood her up - that Harmony loves unicorns, the cuter (cheesier) the better. He breaks the display case and takes it, while the others are still busy feeding on the Magic Shop owner (also tasted like chicken).
He'll leave it in her room - no note. She'll soon guess who it's from and promote him. Then he'll be telling the others what to do.
And once Brad's killed the Slayer - another dumb blonde, so it should be a cinch - he'll dump Harmony, like he did before, and he'll have a ready-made gang of his own.
See? Smart where it counts.