I am sure this has been posted many times before, but why weren't Kelly and Jessica not in the Tori episodes?! Was it a contract or something? I watched a few of those episodes the other day, and I could not the believe the writers would play the viewers for fools the way they did. None of the other cast referred to Kelly and Jessica and it was like they didn't exist. Then, Lisa had the gall to tell Tori, "You're my only best friend." And on top of that, they made Tori Zack's girlfriend! Tori was your typical tomboy, she was pretty, but she wasn't drop dead gorgeous. Zack didn't go for tough girls in leather jackets that didn't have a feminine side. Then when the Tori episodes were over, it was back to Kelly and Jessica like they weren't gone and no mention of Tori again.

Does anyone know what really happened behind the scenes?

Yeah, it's actually pretty common knowledge these days. I'll let Wikipedia do the talking for me:

'The Tori Paradox'

"As mentioned above, the character of Tori Scott (Leanna Creel) was added in later episodes as an initial sparring partner and later girlfriend for Zack. Originally, the final season consisted of thirteen episodes featuring the original cast, and the cast members' contracts with the show expired after those episodes were completed with the final graduation episode. However, NBC ordered more episodes of the show and Thiessen and Berkley refused to sign new contracts. Rather than producing new episodes with only four cast members, a new character, Tori Scott, was added to serve as a replacement for Kelly and Jessie.

Oddly enough, the promos for the final season of the show featured Kelly, Jessie, and Tori together, despite the fact that the actresses never actually appear in an episode together. Rather, the network alternated between "Kelly and Jessie" and "Tori" episodes each week.

During the "Tori" episodes, it was never explained why Jessie and Kelly were not present, and likewise for the "Kelly and Jessie" episodes, no mention was made of Tori. When the season aired in the UK, the episodes tended to be reordered so that the Tori episodes and the Jessie and Kelly episodes were shown together. This was scarcely less confusing, since no mention was ever made of Tori's sudden disappearance and Kelly and Jessie's equally sudden reappearance partway through the season.

Also, during the Tori episodes the school hallway had a different color scheme and Lisa had a different hair style."

Make Wikipedia your friend. It's an extremely valuable resource. :)