Charlie Brown comes up to him and says, "Say, that's sensational, Schroeder...what is it?"
He looks to the reader confused as Schroeder replies, "Grasse Sonata fur das Hommer-Klavier"
As Schroeder resumes his playing, Charlie Brown walks away. The latter says, "Sometimes I feel like I don't belong around here!"
Trivia:
The sheet music shown is indeed the opening measures of Große Sonate für das Hammerklavier (Piano Sonata No. 29) by Beethoven. Schulz took pains to transcribe Schroeder's music accurately.