The first column is the ranking of the schools, by number of estimated deaths. The second column is the name of the school in question (the first school is Beichuan Middle School; most of the schools are primary or middle schools but the #6 school and some others are kindergartens). The third column is the estimated number of deaths, the fourth reflects the number of people whose data has been collected by the project, and the final column reflects the discrepancy.
Update: In his April 28 post, Ai Weiwei reported that 7,605 students died in 124 schools during the May 12th Earthquake. Many smaller villages have not been accounted for, so several hundred might have perished as well; therefore, he currently estimates that around 8,000 students perished that day. Original estimates were around 13,306, but some schools were counted two or three times, thus inflating the number. As of April 26, he has collected 4871 names, and breaks them down by region. He also provides an update as to the areas that have been counted, and lists areas that volunteers need to go to and investigate.
See his original post (in Chinese) here.
See also past CDT posts on Ai Weiwei’s project.