A note my son wrote me 5 years ago when he was a senior in high school.

READ ME!! P.S.- don't get upset with me.:)
HYPOTHESIS: I can be absent and my grades won't suffer and I will not fail.

MY ABSENCES=At least 5
ABSENCES LEFT= approx.13
NUMBER OF DAYS MUST BE present: 162
# of Absences allowed before failing: 18

WORK. x/180
90/100 180 x 90= 16200
100x/100 x =16200/100
x=162
3 to 4 times a semester to be absent

CONCLUSION
Must be present 90% of 180 days of school.
90% of 180 days is 162 days, which means you are allowed 18 absences to pass the grade.
So far, I have had approximately 4-5 absences, may be less, here are my absences list for last semester:
PE 3
GEOM. 2
TH ARTS 1
KEYBOARDING 2

I don't think I have been absent at all this current semester, and if I have, it's been no more than 1 because I remember. So if I have 14 to 13 absences left after the next 2 1/3 semesters, I'm in good shape and my grades aren't slipping, so I can afford further absences.