When I was in school, it was 9 days per year or 4.5 days per semester, excused or unexcused. Even if you missed 0 days in the first half of the year, then missed 6 in the second half, even though it was less than the year maximum, you would still be in danger of being held back. Shows how dumb my school system was. It's been a few years though, so it might be different now.