The Fonz is not happy with his life as just a garage mechanic (even though he is very good at his job.) He realises that one of his greatest regrets is that he never completed high school and so, he decides to enrol again so that he can get his graduate and have his diploma.
Things start out well, in fact, he even earns the admiration of his teacher when he instructs the class to be quiet so that the teacher can speak.
His troubles start when he realises that he is out of his depth with some of the assigned course work, but he figures that's okay because he will just get Richie to do his homework for him!
A very funny episode and one with a moral lesson as we see the pitfalls felt by those who drop out of high school too early and then regret it later on.