One in five children were absent in the first week schools reopened full-time for all pupils, new figures show.
About 85,100 children did not attend on 14 September - the first day schools had to be open to all pupils - an attendance level of 80.8%
Attendance was lower in the previous fortnight because many schools adopted a staggered start to the term.
The unrevised figure by 17 September was 81.2% but statisticians have warned there could be further revisions.
The first pupils started going back to classes at the start of September, with children working in bubbles.
The first day all pupils were expected back in school was 14 September and attendance increased over the course of the week.
Welsh Government data from councils covers pupils of all ages in all maintained schools and nursery settings and will be published daily.