According to Humphrey #7: School Days According to Humphrey by Betty G. Birney (Paperback)

It's the first day of school, and Humphrey can't wait to see his old classmates from last year. But Humphrey winds up cowering in his cage when a group of unfamiliar kids files through the door. Who are these people? What happened to all of his pals from last year?
Humphrey climbs out of his cage to search the school for clues, only to find out that his old friends have moved up to the next grade. Humphrey is sad he won't get to see his old friends every day, but luckily, there's a whole new group of kids to make friends with.

Everyone's favorite classroom pet keeps entertaining kids—and winning awards—across the nation, and his seventh great adventure is no exception!