After Bart switches his test with the Martin (the smartest kid in class), everyone thinks Bart is very intelligent and he is sent to a special school for smart kids. Bart hates it there and tells Principal Skinner that he wants to come baeck to Springfield Elementary so he spy on his classmates and report back to Principal Skinner.
This is considered the first "official" episode.
This episode (and the rest of the episodes in the fist season) have a much longer opening credits than the ones in the seasons afterwards. You will notice that when Homer leaves work there is a man behind him eating a sandwich instead of Mr. Burns and Smithers.
Homer is drinking a beer and then it vanishes when Barney tells him about his job as a store Santa.
The character Lewis changes from black to white.
Barney is blonde in this episode and the rest of the first season.