Although I don't think it ever jumped the shark, here is the consensus from the website.
Jumped The Shark when... Votes
Graduation (The college years) 67
Day One 57
Never Jumped 52
The new class... 31
Tori 16
Kelly, Lisa and Jessie leave the show 11
Jessie's "drug" problem 10
Singing (The Zack Attack) 8
Death (Artie the lizard) 4
Screech works for Belding 3
Hayley Mills leaves the show 3
Berkley gets naked 3
The breakup (Zack and Kelly) 3
Moving (to California) 3
Screech thinks he's Shaggy 2
Special Guest Star (Casey Kasem) 2
Special Guest Star (Tori Spelling) 2
Screech 2
Slater and Zack fight 2
Hawaiian Style TV Movie 1
Puberty (Screech) 1
The kids run the school 1
A second senior prom 1
The roach in the bathroom 1
Puberty (mine) 1
I Do (Zack and Kelly) 1
Recycling 1
The "whoo!!" track 1
Dancing (Slater and Jessie) 1
Peter Engel produces 1
They Did It (Zack and Lisa) 1
Aired too many times a day 1
Lisa never has a boyfriend! 1
Events portrayed in the wrong season 1
Constant Screech abuse 1