In Demand

Friday was the last day for the Seniors at [City Withheld]  High School.
This is a school that I have subbed at many, many times, including a couple long-term jobs.  This year's Seniors were Freshmen when I first started working there.  This is the first time I've ever really been able to see a class through their entire high school career (even if I wasn't there every day). 
Knowing that Friday would be their last day, I decided weeks ago that I would not accept any other jobs for that day. A early last week I started to notice a LOT of job openings.  One school had 10 posted positions for Friday.  I began hearing teachers complaints at other schools about how their posted jobs were not being taken.  I felt reasonably confident that if I did not get into my school of choice, I would easily be able to land a job elsewhere.  There were so many jobs that the staffing agency I work for sent out a mass email telling all their employees to "please make yourselves available" to work on Friday.  That's something I'd never seen before.
Thursday morning I accepted a job at [City Withheld] High School.
When I arrived, I discovered a couple major things.  First, the teacher I was subbing for was actually around and in her classroom all morning, and only needed a sub for the afternoon.  It was unclear why the job had been posted for a full day.  Second, a posted job for a different teacher had gone unfilled.  This kept me very busy all day.

Why was Friday such a popular day for teachers to skip?

Also, one of the Seniors gave me a Senior-photo.  I felt special.