Mr. Belding: Go to class, learn something.
Lisa: Yeah, 'I have all these problems and there's no-one ever there for me. I talk and talk and talk but no-one ever listens.'.
Zack: You know, I've finally found out the best thing about high school, once you graduate you don't have to come back.
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