Several reasons why they left early...

[irehtman]

David Hanson as Chris Atwater
Robert Michael Ryan as Earl Hatfield
Christian Belnavis as Michael Maxwell
Hillary Tuck as Samantha Morgan

Reason:These four acted good, but unfortunately they left after season one is because their talents may have grown too matured quickly.

Kevin Bell as Josh Sanders
Paige Peterson as Amy Wright

Reason: They left after season two it because Paige was acting too out of control and Kevin was acting too dramatic.

Chad Gabriel as Danny Mellon
Reggie Theus as Coach Bill Fuller
Michael Sullivan as Vince D'Amata
Anthony Anderson as Theodore "Teddy" Brodis

Reason: Chad, Michael and Anthony acted good but their talents may have grown too matured quickly. Michael and would have been in this show from the first season so both he and Chad can return as guest stars for the remaining of this show's series. But they don't need to. Anthony would have been appearing in this show from the first season so he can stay until the end of this show's series. Reggie wanted to continue to appear in this show and return as a guest star sometime in later episodes of this show's series , but unfortunately, he had to retire from acting because he was acting too dramatic and wanted to stick on NBA coaching forever.

Mark Famiglietti as Nick Hammer
James Villani as Rick Bocso

Reason: These had some talent, but they were both considered the worst actors in this show's history. Mark left because he was acting too dramatic even if he guest starred once in both season five and season six. James was acting too lazy, indolent and reluctant, so he retired from acting.

I know it is not right to change casts too much, but there are reasons why. Either bad acting or too matured acting. But at least these too many cast changes are not bad in this show because 1.) they are even-numbered rather than odd-numbered and 2.) Daniella and Megan are the only constant main and supporting actresses appearing in all episodes and seasons of this show.

[TMC]

I just came here to ask about why there were so many cast changes over the six years that Hang Time was on the air? At the end of the day, only Daniella Deutscher (Julie) and Megan Parlen (Mary Beth) were on from beginning to end. Another Peter Engel show, Saved by the Bell: The New Class had the same sort of problem.

[irehtman]

Originally Posted by TMC

I just came here to ask about why there were so many cast changes over the six years that Hang Time was on the air? At the end of the day, only Daniella Deutscher (Julie) and Megan Parlen (Mary Beth) were on from beginning to end. Another Peter Engel show, Saved by the Bell: The New Class had the same sort of problem.

The too many cast changes in Hang Time aren't as worse as the too many cast changes in the New Class. The other reason why is because Julie is the main character and Mary Beth is the funniest supporting character and that's tolerable enough for Hang Time not to get turned down completely, TMC.

[TMC]

This is from Dustin Diamond's supposedly, largely ghost written book Behind the Bell, so take this with a huge grain of salt. The book says that Peter Engel, the executive producer of Hang Time, was unforgiving when it came to money. This is why Kelly and Jessie didn’t return for the final episodes of Saved by the Bell. This is also presumably why, there was such a rotating cast on Saved by the Bell: The New Class.

[irehtman]

Originally Posted by TMC

This is from Dustin Diamond's supposedly, largely ghost written book Behind the Bell, so take this with a huge grain of salt. The book says that Peter Engel, the executive producer of Hang Time, was unforgiving when it came to money. This is why Kelly and Jessie didn’t return for the final episodes of Saved by the Bell. This is also presumably why, there was such a rotating cast on Saved by the Bell: The New Class.

You also forgot to mention about this missing info of "Behind The Bell" book about original SBTB creator Sam Bobrick. The Hang Time creators are not permanently problematic as Bobrick, just to give you a heads up.