When a student’s annual attendance drops below 90%, they become a persistent absentee. You can see a list of students who are persistent absentees from Students > Attendance > Absentees > Persistent Absentees. We have more information here that you can take a look at.
Their attendance can improve throughout the year and if it goes above 90% again, they will no longer be classed as a Persistent Absentee.
In a normal school year, the DfE says that students should attend at least 380 sessions or 190 days. You can take a look at the DfE's guidance on page 17 here.
Therefore, students who miss 38 sessions/19 days or more throughout the year will be classed as a Persistent Absentee for that academic year (as their attendance drops below 90%).
There are two sessions in each school day - morning (AM) and afternoon (PM). The times of these sessions are determined by your school's roll call times. You can take a look at how to view your roll call times here.