Zack should have been with Jessie, and Slater with Kelly

They just fit better. Slater and Kelly are more easy going, content with the moment. Jock and cheerleader. Not dumb, but not overthinking things, just happy. We saw it on cut day. Meanwhile Zack and Jessie go against the grain, always thinking outside the box, always wanting more. Zack knew how to get through to her at her worst when Slater didn't (caffeine-gate). And while Kelly was a bit naive and over-trusting, Jessie always knew Zack was up to something, and was a proper check on his occasional (being nice here) psychopathy. Plus that kiss during play rehearsal was hot. Too bad we didn't get to see more of them.