This came on Comedy Central back in the mid 90s. It had 2 girls (maybe sisters) going to a boarding house school. One girl was the rebellious and "Wacky" girl. I just remember her pretending to be pregnant during a graduation. It was really weird. And I think there was a camero by an old horror actor like Christopher Plummer or something. Please help me.
It was very possibly Private School (http://www.imdb.com/title/tt0086143/). It's been a while since I've seen it, but it fits your description fairly well from what I can recall.