They definitely do a good job on those sites. Like some others said, it's all about casting fresh faced amateur girls, many of whom only wind up doing a couple of shoots. Several of the amateur girls I recruited for the teen party group sex scenes featuring American girls on
http://SpicyRoulette.com only did one or two shoots and disappeared and that makes it more realistic.
And of course one needs to create realistic looking sets with bunk beds, and giving them a lived in look. I've had sets created and shot dorm room scenes like this on a more limited basis for a single adult movie or a couple of scenes for a site. But you need a nicely set up studio space with multiple sets, furniture and lots of props to do an ongoing series of scenes with a lot of people in it.
I already have access to space in a porn friendly studio where I have shot before that can accomodate this time of production and l'd love to take on a project like this if someone had the budget. I'm always casting enough new girls each month that the turnover of new faces is very high.