JW's always sat down when the National Anthem played. If they were already sitting they would remain seated.
Then there was a change, 1985 I think, that said if you were already standing you did not need to sit but could remain standing during the Anthem.
George
Edit: - school brochure
*** sj pp. 15-16 Flag Salute, Anthems and Voting ***
When national anthems are played, usually all that a person has to do to show that he shares the sentiments of the song is to stand up. In such cases, Witness youths remain seated. However, if our youths are already standing when the national anthem is played, they would not have to take the special action of sitting down; it is not as though they had specifically stood up for the anthem. On the other hand, if a group are expected to stand and sing, then our young people may rise and stand out of respect. But they would show that they do not share the sentiments of the song by refraining from singing.