"Bart Gets a "Z"" is the second episode of Season 21.
When Mrs. Krabappel confiscates her students's mobile phones, they hatch a plan to get their teacher drunk to get her to "loosen up". When Principal Skinner fires her, she is replaced by a cool new teacher, Zachary Vaughn, who impresses the kids with his love of texting, Facebook and Twitter. Bart has a lingering guilt about getting Edna fired, and tries to help her. With the assistance of a self-help book-slash-DVD, she opens her own muffin store - but she still plans to be a teacher.
Bart and his classmates get their cellphones confiscated by Mrs. Krabappel for using them in class. At a meeting in Bart's house, the students realize that Homer is always fun to be around when he's drunk so they plan to get Mrs. Krabappel drunk, in an effort to make her more fun. Each student steals some alcohol from their parents and puts it in Mrs. Krabappel's coffee. After Mrs. Krabappel gets drunk she makes a scene and it results in her getting fired. Bart feels bad, so he plans to help Mrs. Krabappel get back on her feet. Bart and Milhouse buy a book called "The Answer". Following the instructions of the book, Bart and Mrs. Krabappel build a Muffin store called Edna's Edibles. That's when Bart confesses that he was the one who put alcohol in Mrs. Krabappel's coffee. Mrs. Krabappel is furious at Bart and takes off.
Meanwhile, at Springfield Elementary, a new teacher called Zachary Vaughn teaches Bart's class and gives everyone their cellphones back, everyone loves him, except Bart. Bart plans to put alcohol in Zachary's drink as well, but backs out at the last second and chooses to tell Principal Skinner instead. While he is confessing, he and Skinner hear someone yelling in the hallway. It is Zachary, shouting drunkenly that he hates children. Skinner initially thinks Bart spiked his drink, and states that he thought Bart told him that he didn't spike it. Zachary confirms Bart's statement, as he has a tendency to spike his own drink. Willie takes him away (muttering that it always happens to the good ones as he does so), although not before Zachary attempts to defame the faculty by claiming that they just want to control the students. Mrs. Krabappel gets her job back, although she does give her revenge to the students for causing her to get fired in the first place by giving them extremely stale muffins.