Juniors and seniors in a pledge class isn't odd in and of itself especially at schools where the Greek system isn't huge. We had juniors in every semester's pledge class when I was in school and also seniors. Considering 4 years is not the norm anymore, it's not surprising.
I think the Sigmas though, felt like there were some of the older girls that would never have come out had it not been for the cameras. I don't think MTV had to bother asking people to come out. There are enough hams out there ready willing and able
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Since this is the first time they had a pledge house, they're going to have to plan some things quickly - cut them some slack. They never said that the pledges can't drink, they just cannot do it in the pledge house. Same rules as a regular sorority house. I would assume if the other pledges come over or stay over, they have to follow the rules, just as sisters who live out of the house do when they come to the house.