Revealing mistake: The graduation stage appears to be set in front of a woodland area. However, when Kel climbs out of the bulldozer after having demolished the stage, you can see that it's actually a backdrop as the rail holding it up is visible.
Aw, Here It Goes To Hollywood (2) - S4-E9
Continuity mistake: When Kenan and Kel are chased out of the shower by David, the clothes that he's wearing are completely dry, despite the fact he was just in a running shower.
Other mistake: When Roger writes the note after Kel glues his mouth shut, Kenan reads back far more than Roger could realistically have written in the short amount of time he took to write it.
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.