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.
Other mistake: In the opening credits of Season 4 (the Tori episodes), when Leanna Creel is shown, the second of her three sequence shots is in reverse (where she is sitting at the Max in a red outfit in front of a candle). In the episode where that clip is from, she was sitting to the right of the table facing left, but in the opening sequence, she is on the left of it facing right (mirrored image).
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