Fears of families having to self-isolate over Christmas has prompted some parents to pull their children out of school before the end of term.
Most Welsh local authorities have said schools should remain open until 18 December, despite calls from some unions to end lessons early.
Pupils in Blaenau Gwent will have their last day in school on Wednesday before moving online until the 18 December.
The area recorded the highest Covid-19 infection rates in Wales last week.
Families with children have been told they should consider "pre-isolating" at home for 10 days before Christmas if they are planning to see elderly relatives, in a report by the Welsh Government's Technical Advisory Group (TAG) on Tuesday.
But Health Minister Vaughan Gething said harm had been done to children when schools were closed, particularly vulnerable children, highlighting the mental health impact particularly among secondary school age children.