I asked that very question at the PTA meeting the other night, and you would have thought I asked them what day was naked day for the kids, from the look I got.
ALthough I was not on the calendar committee for Jo. Co. Schools last year (which created this year's calendar), I served on it a few years ago. Each quarter needs to be 45 days long since some middle schools have elective classes taught by quarters. This makes the classes equal for each grading period. Also, the school system tries not to have Thursdays off since many parents would then take their children on vacations for a 4 day weekend. This leads to very high absenteeism on that Friday. Part of the school report card and No Child Left Behind is judgement on the number of absences during the school year.