Why is Grover bullied by the other students?

Grover was bullied because he was different, and his differences made it easy for other kids to bully him.

He was scrawny, emotional, cripple (a muscular disease in his legs), and he walked funny. Grover was an easy target. He was scrawny. He cried when he got frustrated. He must've been held back several grades, because he was the only sixth grader with acne and the start of a wispy beard on his chin. On top of all that, he was crippled. He had a note excusing him from PE for the rest of his life because he had some kind of muscular disease in his legs. He walked funny, like every step hurt him, but don't let that fool you. You should've seen him run when it was enchilada day in the cafeteria.