Continuity mistake: When Zack is in middle school, his teacher, Miss Bliss wants to talk to his father. At that time his father's name is Peter Morris and he is an older man and unmarried. When Zack is in high school and Mr. Belding wants to talk to him about the ski trip, his name is Derrik Morris and he has a mother.
Continuity mistake: In the scene where Max has Zack bring Kelly a Coke, you can see the glass only has one straw in it. You can verify that it's simply not two straws stuck together when he lays it on Kelly's table. As he lays it down, the straw swoops around the outer perimeter of the glass and it can be verified that it's only one straw. However, when Kelly decides to take a drink of it there are now two straws in the glass.
Continuity mistake: When the teacher is going through the study fast, you see Kelly stretch her wrist as it hurts from writing so fast. While she is stretching, behind her you see Zack putting on his headphones. A few seconds later the camera is on him and he is putting the headphones on again.
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