They had to pull together Undeclared really quick. Some of the planning happened while Freaks and Geeks was still airing, so most of the cast was stuck contracted to F&G, so he couldn't reuse as many of them as he wanted.
Where did you hear that?
Either in the commentaries or the booklet with the DVD set. I probably got some details wrong.
From what I recall, the people they did get for the main cast, they were working on such a tight schedule that there was little time for development, so most everyone's character is actually based off of themself. It was also very ad lib heavy, at least for the first batch of 6-7 eps.