Visible crew/equipment: In the scene where Zack talks to Jessie about switching desks to be next to Kelly, you can see the top of the set for quite a few seconds including lighting, scaffolding and tops of the adjoining rooms. (00:06:15)
Visible crew/equipment: During the opening credits, as Zack Morris walks into the frame of the school store, you can see the boom microphone briefly lower into the shot up near the EXIT sign on the wall. (00:01:20)
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.
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