There are a total of 90 reported absences for the week. To find how many students this involves, we subtract the 8 that are counted twice, and 2 for the one student that was counted 3 times. This means 90 - (8 + 2) = 80 students were involved in the absences. So, we are left with 80 is 5% of what number. Since 5% is 1/20, we find the number of students in the school is 20·80 = 1600.