Max did that stunt to impress her, but she didn't know that it was Max until the principal exposed him. If the principal hadn't interrupted the stunt, would've Max revealed his identity to the crowd? Also, I don't get why Max is all like "I blew my one chance to impress Roxanne" when he's at the principal office. Even if he didn't finish the performance, he still accomplished a lot (defied authority, did lip-syncing perfectly, showed off his dance moves, made everyone cheer, and flirted with Roxanne)