A situation like this happened with my sorority last spring.. except we were the ones now showing up.
Our social planned a mixer with a frat during finals week, and naturally a lot of the girls were studying. Only about 7 girls showed up, but most of the guys were there. The frat was PISSED and had their pledges egg our house that night.
We're OK with the frat now after our social explained that most of the girls were studying for finals. They still never apologized for egging our house though..