The blaze broke out at Coed Eva Primary School, in Cwmbran, at about 02:25 GMT on New Year's Day.
Most pupils returned on Monday, though nursery children will have to wait until 25 January when temporary classrooms will have been installed.
Six people, including four teenage boys, arrested in connection with the fire have been released on bailed.
An infants and nursery school in Torfaen has reopened 10 days after it was gutted by fire.