I've asked JCPS and the board to provide exact numbers and explain why so many teachers are attending professional development conferences on a school day.
It seems strange that so many teachers would take off on a single day. I found a 2007 survey that indicates the average unscheduled absenteeism rate in business is around 2.5 %, which appear to be well under the rate reported JCPS Friday if we assume at least 400 were out. A combination of forces that may have taken 10% or more of the staff out of the classroom is something that JCPS needs to explain with something more than guessing it was caused by the flu.