It's only half a day, but I've never taken them out of school for anything except completely immovable medical appointments.
DSs in yr 8 & 10, both in Army cadets. They've been asked by their commanding officer to form part of a colour party and do a short remembrance service at the junior school they went to, which would mean missing morning lessons on 11 Nov.
Would you let them do it? What are the chances of the school approving it?