Ponyboy didn't want anyone at school to notice he had broken his arm. After all, it wouldn't be cool if a big deal was made of it. The Soc who broke his arm didn't know that he had broken Ponyboy's arm, and so if he did find out chances were Ponyboy would be targeted more. After all, a Greaser with a broken arm? It was like hunting season for them. 'Hahaha, Greaser season...'
But of course, when you're friends with Two-Bit, nothing can be kept a secret for long.