As most of you know, Monsters University is set before Monsters Inc while Mike and Sulley are in college. The trailer shows the two scarers-in-training at a frat house, where Sulley takes advantage of a moment when Mike is asleep to turn Mike into a disco ball for the frat party.
What you don't know is that the frat house that the two college boys join hosts a scaring competition, much like Greek Games in our world of college fraternities. They all have to organize a group of monsters and prepare them to be scary.
Billy Crystal (voice of Mike Wazowski) mentioned that Mike and Sulley are actually only 17 and 18 years old in the prequel, and are far from being the big men on campus. It can kind of be assumed for Mike since he's wearing a retainer in the preview.
Monsters University is released on June 21, 2013.
In case you haven't seen it yet, here's the trailer!