Many Chicago area schools have Spring Break toward the end of March. In 2004, our Spring Break will begin at the end of the school day on Friday, March 26. The school holidays are March 29 through April 2.
As this thread shows, there's no such thing as a Sping Break week that all US schools observe. It seems that some schools base their Spring Break on Easter, observing either the week before or after. Other schools schedule Spring Break to fall neatly between school quarters. Still other schools just have a set formula, such as "3rd full week of March."
From the standpoint of crowds on the ship, I really don't think it makes any difference. I assume that all March and April sailings go out with close to 100% of the staterooms occupied, and with a similar mix of adults and children. The ship is designed to handle full occupancy, so crowds should not be an problem.
It's just that the folks who are sailing right aound Easter are paying more, regardless of whether they are using DVC points or paying cash.