Recently at Muppet Wiki I had asked a question about wether anybody besides Roscoe Orman voiced Hard Head Henry Harris. While nobody could realy tell, a link to the Roosevelt Franklin sketch about poision was posted on Hard Head Henry Harris' talk page, and there I founf video clips of all three Roosevelt Franklin Elementary School sketches that were shown on Noggin.
It seems like two of those sketches, the poison sketch and the lous and soft sketch, were really short, while the other sketch, where Roosevelt talks about africa, is longer. While studying those, the first two sketches mentioned doesn't have much focus on Roosevelt's classmates. They msotly talk at once, with some only saying a few words, and the sketches were short and more to the point, while in the africa sketch, Smart Tina and Hard Head Henry Harris were more prominently featured. I also noticed that in the last sketch I mentioned, during the theme song as the exterior of the school is shown there is a faded image of the class singing, while in the other two there's just an outside shot of the school. This leads me to think that these sketches might have started out as being primarly sketches starring Roosevelt, and then in a year or two they started developing his classmates, though most of them had appeared.
I can't tell who voiced Hard Head Henry Harris in the poision sketch (he speaks very briefly, and this sketch was seen in episode 514, from the fourth season, two years before Roscoe Orman took over the role of Gordon), but it does sound similar to his voice in the africa skecth. Of course, I can't tell any similarities between Hard Head Henry Harris and Gordon's voices, while Smart Tina does sound a lot like Maria. Then again, I don't think Roosevelt Franklin sounds like the original Gordon, and I don't think that Same Sound Brown sounds like David.
And while most of the classmates were voiced by human actors, the regular performers clearly sang the theme song to the sketches. I could very clearly hear Frank Oz and Jerry Nelson, and at one point I heard a Jim Henson voice.
Interestingly, in the poison sketch, when Smart Tina talks, she sounds more like Susan than Maria (unless I am mistaking the character with someone who looks very similar).