Continuity mistake: When Tori joins the group during the final season, she and Zack date for two episodes, but they are not in sequential order.
Visible crew/equipment: After the fashion show when Zack says "Sorry Lisa, I don't know what to say" an entire boom microphone assembly is visible and snatched out of view at the top of the screen.
Continuity mistake: In the 1st season (the Ms. Bliss episodes), Zack mentioned to Screech in one episode that he had good news for Screech's brother. But in the Bayside episodes, Screech confirmed being an only child (specifically said in this episode).
School Song - S4-E24
Continuity mistake: Zack messes with the piano so it will give him a chance of winning. When Tori plays it sounds like she is messing up. But immediately following that screech plays the piano with no problem and there was no time to fix the piano.
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