Once Upon a Frog by Sarah Mlynowski
Whatever After continues. Spoilers for the earlier works ahead.
In particular, Jonah's funny memories of things that happened to Maryrose.
But, meanwhile, a boy's being nasty in school, and when Jonah and Prince go through the mirror, Abby follows and finds herself in a well with a talking frog.
Then Prince prevents a golden ball from going into the well.
Resolving this includes guards who aren't very good at it, a diary, throwing against a wall, a servant in a blue hat, and more.