It is middle school aged, maybe 5th and 6th grade, and very much like a Beverly Cleary book, and it was in the mid to late 60’s that I read it. You flipped it upside down to read the alternate ending, which had a different cover. I loved it back then because it showed that bullies and the bullied are not so much different. The boy had a major challenge…to play in a school concert, it was either a bugle or a trumpet, and that was the end of both versions of the story.
I’ve looked all over and it seems not to be Beverly Cleary. I know I read it about the same time I was reading Beezus and Ramona.
Are you sure that this was one book, or could it be two parallel books telling the same story from the point of view of a bully and his victim? Because if the latter is possible, it sounds very much like Mary Stolz’ ‘A Dog on Barkham Street’ and ‘The Bully of Barkham Street’. Edward has problems with his longing for a dog; his delightful but unreliable Uncle Josh; and being threatened by the local bully Martin. Martin has his own problems. The books do end with a school concert, where Martin has to blow a bugle.