There is a website that lists the majority of colleges and when they have spring break. I only looked at it quickly but it seems like the week of March 5th is the most popular.
Navigator of the Seas- Nov. 2007
Enchantment of the Seas- Jan. 2008
Grandeur of the Seas- Feb. 2008
Fascination- Jan. 2009
Triumph- Jun. 2009
Enchantment of the Seas- Aug. 2009
Grandeur of the Seas- Dec. 2009