So this is why Jessie and Kelly dissapeared in the 5th season of Saved By The Bell.
When Kelly and Jessie (Tiffani-Amber Thiessen and Elizabeth Berkley) left the show to pursue their own interests, the producers of SBTB pretty much attempted to "pull the wool over our eyes" by offering NO EXPLANATION for Kelly and Jessie's absence. The only mention of Kelly and Jessie we get is when Zack "talks to them on the phone" during the episode where he takes a day off of school and reminisces about the greatest summer of his life at Malibu Sands.
Otherwise, it's assumed that "Kelly and Jessie" were at Bayside the entire time that Tori was there, they were always just "off camera". Once the Tori episodes were finished airing, the producers showed the "final" graduation episode with Kelly and Jessie included (because it was already filmed when the two were still part of the cast).