Saved by the Bell/WMG

The entire series takes place inside the school
X-Entertainment goes into more detail, but the gist of it is that there are a huge number of inconsistencies and pieces of illogic that make "sense" if you accept that everything shown onscreen in the series takes place in school, except for the one episode where Slater and Zack get drunk drinking a sip of alcohol.

The Tori episodes are a self-insert fanfic on Tori's part.
The obvious new girl in school, she has imagined herself into the main circle of friends. The first batch of final season episodes, with Jessie and Kelly, is the canonical one. The switch occurs when Zack talks with the two over the phone in he summer flashback episode.
 * Y'know, I'm really glad I'm not the only one who's always felt that those episodes felt like a Mary Sue self-insert fanfic. It would totally have been plausible too. I mean think about it this way: she writes the fic with Kelly and Jessie wiped out of existence (thus eliminating her main competition for Zack and Slater). Lisa, the remaining female main, is then written to gush about how awesome Tori is and actually plead with Tori to be her buddy in the first "Tori episode." Then, Tori writes Zack and Slater as falling madly in love with her, and even handwaves Screech's Lisa-obsession away and makes him fall in love with her too, y'know just to show how desirable she supposedly is. Yes, it would have been perfect if the final "Tori episode" had ended by showing Tori sitting alone at a typewriter, revealing that the entire bizarre arc was just a fanfic.

Zach is one of the Q.
As in from Star Trek: The Next Generation. He can stop time, break down the fourth wall, and date the head cheerleader. How else could he pull it off?
 * Time Lord?
 * Marty Stu!

Kelly drowned between high school and college, and Zack's delusions got progressively worse until he was institutionalized around the Kelly tries to leave on the ship/going to Reno arc.
They were vacationing on Zack's father's yacht, when the weather started getting rough, and the tiny ship was tossed. Since there was no fearless crew, the Guppy had been lost... along with Kelly's life. The "kelly" who walked in, ruining his little story, was a different Put on a Bus character from earlier in the series, and there was no class that sent sophomores to ive on a luxury cruise liner for a year and a half; that was his parents coming to get him and lock him away for his own safety, after repeated drowning attempts. The final episode is after he manages to drown himself in a bowl of chilled soup, finally being reuinited with Kelly in the marriage they had discussed after patching up their differences on the yacht years before.

This entire series takes place in the alternate universe created by Enrico Pucci in Jojo's Bizarre Adventure Part VI
Incidentally, Zach is Dio; he has almost the same hairstyle and hair colour as Dio, he can stop time, and rules don't apply to him. He even has two unique catchphrases: Dio's "Toki wo tomare!" and "Soshite toki wa ugoki dasu" are replaced by "Time out!" and "Time in!". Screech is probably Jonathan Joestar, kind and intellectual, but inferior to Dio. He doesn't even have a stand, just intelligence and ripple powers. Slater is definitely Polnareff - a powerful idiot who hates Dio.

== Saved by the Bell, That's So Raven, Hannah Montana and most, if not all of Disney Channel's other live action shows (except Lizzie McGuire, possibly) all take place in the same universe. == Bayside High was remodeled at the turn of the millennium, which leads to the start of That's So Raven. Combine that with the two crossovers that Disney Channel aired in 2006 and 2009 and it makes sense. Palisades could be a neighborhood in San Francisco, if the whole "technically that can't ever be possible" argument is made. And, to top it all off, Good Morning, Miss Bliss aired on Disney Channel before NBC retooled it, so the connection is probably somewhat intentional.

Zack is a mythological Trickster figure, who has been forced by the Gods to become a teenager and go through high school
Many mythologies contained an archetypical roguish-but-well-intentioned Trickster figure such as Anansi or Coyote. It turns out that Zack is one of these figures as well, but one of his pranks apparently went too far (sometime during the mid-1980's), and the Gods decided to punish him by turning him into a teenager and go to school like any human being. Think about it: Zack has the classic Trickster personality, plus the inexplicable ability to freeze time.
 * First, the Gods created the name/identity "Zack Morris" for the Trickster, and placed him as a pre-teen in Indiana, at JFK middle school with Miss Bliss. Eventually, they decided that JFK middle school wasn't bizarre enough, so the Gods literally retconned that away, and relocated the entity now known as "Zack" to the Palisades neighborhood in Los Angeles. They also decided to transport Mr. Belding, Screech Powers, and Lisa Turtle to LA with Zack, because they felt that these mortals had potential to teach him about life (or something).
 * After relocating Zack and those 3 mortals to Bayside High, the Gods apparently felt satisfied that this was a better fit than JFK middle school, and let the story proceed. Any and all continuity errors were the result of the Gods retconning certain details in order to craft the scenario that they wanted. Throughout his new life at Bayside, Zack/Trickster still created all kinds of zany schemes and froze time, as was his way, but also interacted with the other characters, and gradually formed true friendships and learned little things about life from them. He even fell in love with the mortal girl Kelly Kapowski.
 * By the time of The College Years, Zack was no longer freezing time (although he still came up with pranks and zany-yet-harmless schemes). This is because in the process of living among the mortals of Bayside and falling in love with Kelly, he had finally decided to become human permanently so that he could be with Kelly. Zack had finally learned humility, and the Gods were pleased.

The Great Lakes Teen Show Postulate.
If the show had stayed "in" northern Indiana it would be both better accepted by critics then and more fondly remembered as something other than a cheese-fest now. Just like Freaks and Geeks (Detroit), Degrassi (Toronto), Glee (northern Ohio) and That '70s Show (southeast Wisconsin).

The reason Zack, Screech, Lisa, and Mr. Belding moved from Indiana to California because they witnessed Miss Bliss' murder
Possibly a mafia related shooting or something. They were put into witness protection, but they ended up making the proper arrests sooner than expected. Rather than move back to Indiana, their families decided to settle down where the government placed them.
 * It also explains why Zack and the others don't talk about those days much. Because they want to forget about it and because they're afraid they might end up being targeted again somehow.
 * Maybe Miss Bliss was killed by that nutty science teacher who got so infuriated by the fact that "one of YOUR students" (nevermind the fact that Nikki was also his student as well) freed the frogs from the science lab. C'mon, that guy just exuded insanity, it's not that hard to imagine him being the one who killed Miss Bliss.

TORI was in the witness protection program

 * She just showed up out of the blue one day during their senior year. This was because of the suddenness of her entering the witness protection program and being quickly relocated to the Pacific Palisades neighborhood of LA. The fact that she constantly seemed on edge when she first arrived was because she was still terrified that whoever was chasing her might find her.
 * The reason she rode a motorcycle was so that she would be able to make a quick getaway if it ever became clear that her pursuers had found her. Evidently this became the case towards the end of senior year, as Tori suddenly disappeared just as suddenly as she had appeared.

Executive Meddling forced the Saved by the Bell writers to create a character (Violet) for Tori Spelling

 * It's well-known that her father, the rich and powerful TV producer Aaron Spelling, was the driving force behind her career. It's entirely possible that he used his clout to force NBC and the SBTB crew to create a guest-starring role for his daughter. This would explain why in "The College Years," the show eagerly had Alex make a Take That towards Tori Spelling in one episode; they resented being arm-twisted and wanted to get in an insult once she was gone.

The events of the Zack Attack episode were not just a Dream Sequence. They really did occur in an Alternate Universe...
...and the reason Zack is dreaming about it is because the residents of that universe...having apparently gone through years of pain and turmoil before setting things right...built a device to beam their history into the mind of our world's Zack in order to warn him not to make the same mistakes they made. The reason they were able to build such a device is because A: The wealth that they gained from being pop stars gave them the resources, and B: Because in their universe, Screech never lost his Mad Scientist skills.

OR....

Similar to the above theory: the events of the Zack Attack episode were not just Dream Sequence, but instead of being an Alternate Universe, they were from a Dark Future
And the future!Zack and friends built the above-described device to beam the memory of their history back in time to early 90's Zack in order to make him have the Future Me Scares Me reaction and thus prevent him from making the decisions that led to their Dark Future, in order to Set Right What Once Went Wrong.

And just in case that didn't work, their Plan B is to personally travel back in time and assassinate Mindy the evil music exec before she gets a chance to corrupt Zack.