The Mamas and the Papas - S1-E12
Character mistake: Lisa is upset about being paired up with Screech. Mr. Belding explains to Lisa that the matches were paired up alphabetically. But this is proven incorrect - although Kapowsky (Kelly) and Morris (Zack) are very close, and Slater (AC) and Spano (Jessie) are even closer alphabetically, Powers (Screech) and Turtle (Lisa) should not have been paired. Assuming no other kids names' interfered with these six (in which case the groups would be different anyway). The correct pairs would be Kelly and Zack, Screech and Jessie, then AC and Lisa according to true alphabetical order last names.
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