This is book number 2 in the Humphrey (Audio) series.
After the holidays, Humphrey is shocked by a big surprise in Room 26-a new class pet Humphrey tries to be welcoming, but Og the frog doesn't respond to any of his friendly squeaks or visits (remember, he has a lock-that-doesn't-lock). Plus, the students are so interested in Og, they almost stop paying attention to Humphrey altogether. Humphrey doesn't like the mad-bad-sad feelings he's had since Og came, but luckily he still gets to have adventures with different kids on the weekends. Friendship can be tricky, but Humphrey is an intrepid problem-solver. If any hamster can become buddies with a frog, he can.