“Bart Gets a ‘Z’” is the second episode of Season 21, which aired on October 4, 2009. The episode was written by Matt Selman, and directed by Mark Kirkland.
When Mrs Krabappel confiscates her students’ mobile phones, they hatch a plan to get their teacher drunk to get revenge. 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 longs to be a teacher.