The basic theme of the show is a retelling of Pinocchio with a twist. Leonard Helperman's mom is also the teacher of his 4th grade class, so he is already as far from normal as possible (esp. since his mother uses silly language like "right-a-rooney" - she is voiced by That 70's Show's Debra Jo Rupp).
In the very first episode, his dog Spot (voiced by Nathan Lane) decides that he wants to go to school too. Against the wishes of Pretty Boy (the canary - Jerry Stiller) and Mr. Jolly (the cat - David Ogden Stiers) who want him just to be a lazy soap opera-watching, internet-surfing pet like them. But Spot dons Leonard's clothes, and wears a beanie on his head to hide his ears. He goes to school and finds Mrs. Helperman's class were he says he is a new student. When he says his name is "Spot" Mrs. Helperman thinks he says "Scott" and then he finds his last name, Leadready II on her pencil. From that day on, he becomes the head of the class - constantly going above and beyond and basically becoming the Teacher's Pet (in more ways than one). In classic Nathan Lane style, he will often jump up on his desk to give an answer to a question, or the similar. He's the most popular kid in class too - much to the dismay of Leonard.
Each ensuing episode focuses on Scott/Spot and Leonard and 4th grade life as well as trying to constantly keep Scott's true identity a secret. For example, Scott has to cross-dress to pass himself off as his mom for Teacher-Parent night. And when it's Parent Profession day, Scott has problems when a mailman shows up. These episodes are also filled with the antics of Mr. Jolly and Pretty Boy. The episodes are very fun but also very intelligent - something rare in Saturday-morning cartoons.
One other thing worth mentioning to make the movie more enjoyable... Ian Wazeluski is referred to as "Eeu-an" by his fellow classmates for good reason. He does the most random, grossest things ever. For a report on the Odyssey, he made a model of a ship out of ear wax. Just keep that in mind.
Hope this helps - and if you need anymore info - just let me know!
Jessica