Save the Max - S2-E3
Factual error: When Zack is given the old newspaper with Mr. Belding's article, the headline reads, "Picture on pages 3 and 4." When he looks at the picture, he looks at the middle of the paper, with quite a few pages at both sides of the newspaper. Pages 3 and 4 would be near the front and would be on opposite sides of the same page.
Save the Max - S2-E3
Revealing mistake: When they are reading the reviews in the newspaper, look at the back page. It's the sports section. Next when Jessie comes back to Zack and tells him what Mr Belding was hiding, she says the paper is from 1968. But look at the back of that paper also. They are using the exact same paper. The reviews and the 1968 paper are the exact same.
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