The Wicked Stepbrother (2) - S3-E14
Character mistake: While holding the fake raffle, Zack says Mr. Belding won't be back for two days. Mr. Belding shows up suddenly, surprising the boys that he's back early - however, at the end of the scene, Slater says to Belding, "we weren't expecting you back til tomorrow."
Character mistake: Brandon begins the scene giving his speech while he is sitting/leaning on the desk. The students all move into the picture from both sides and gather around Brandon then they all say, "There's No Hope With Dope". After the conversation with Mr. Belding, they watch the commercial they just shot (beginning with each student giving his/her opinion about drugs). Then Zach says his speech while sitting/leaning on the desk. He stands up, the rest of the students and Brandon move in and gather around Zach, then Brandon gives his anti-drug speech.
Answer: As far as I can tell Jessie and Kelly decided to leave the show because their contracts were up and they wanted to pursue other things. But there were still episodes of Saved by the Bell left to tape so Tori came along to fill that void. Jessie and Kelly were supposedly still at Bayside, but they were "off-screen." The Graduation episode was filmed before Kelly and Jessie left the show.
ACertainShadeofGreen