What does missing “a lot” of school mean?

“Chronic absenteeism” is when a student misses 18 or more full days of school (excused or unexcused). That could be as few as 2 days a month. But it adds up. Educators and parents have looked at data and see that missing more than 9 days of school each year can make it hard to catch up and keep up. Missing 18 days a year can make it even harder, and students risk falling behind in reading and math, and face a hard climb to graduation.