I'm looking for an animated film I saw as a child.

The film was about a boy who encounters a series of monsters on his way to school in various guises (a swimming pool, a girl, a blue blob with arms and legs, and something else I don't quite remember. Each of the monsters urges him to perform an action that is wrong, and he refuses each time. Each time, the monster meets a different fate when he refuses their offer (falling down dead, running away, fading away). Previously, I had done a page on this film, using the first line of the song incorporated into the film as its dialogue ("I know the monsters/Who live beneath the sea./I know the Monsters/And the monsters they know me!") as its title, but I now think I might be mistaken as to the title. Can you help me out?