http://r4d.dfid.gov.uk/pdf/outputs/systematicreviews/Q39Teac...
The primary reasons for absenteeism seem to be personal/family health (which, granted, that could be bullshit if it was self-reported) and work related requirements (training, etc.)
Several studies showed correlation between poverty and absenteeism, which suggests to me that teachers who have less financial resources to deal with personal issues are going to miss work more often to go deal with those issues.
It was also noted that administrators were absent more often than teachers and the author suggested this as being a factor in teacher absenteeism.