Last year, we went the last 2 weeks of March & it was very busy. The line for a fast pass on Toy Story Mania was 90 minutes at 20 after the park opened. We asked one of the CM if this was really busy and they all said nothing compared to the week between Christmas and New Year. Last year, too, the weather was unseasonably cool. Week 1 was awful, but week 2 was much better with smaller crowds and warmer weather.
We also went during that time in 2008 and it was great. We are going again this year. It looks like the schools are all over the calendar as to when they are having breaks this year, which may spread out some crowds.