I know where you're coming from, and appreciate your annoyance - but for whoever's charged with overseeing attendance, it's a really tricky one - they're in a 'damned if you do, damned if you don't' situation. Yes, schools are under the cosh over attendance. Yes, they have to get figures up to (what I personally think are) increasingly unrealistic figures. I know, and believe you when you say that you've been supportive, and strict about attendance in the past, and are miffed on this occasion. But, the flip side of this is that, if it's like most other DofE events, staff will likely have given up free time; worked out of hours, overseen a demanding event and pitched up for school the next day. Of the kids involved, I'm imagining a significant proportion of the kids who took part in the event then didn't show up for school the next day, and I think that this is where your problem lies.
One case I can remember concerned a weekend trip abroad which came back at 7pm, and a significant number of pupils simply didn't turn up for lessons the next day. The teacher who ran it had sacrificed a weekend with their own young children to take the group. Those who didn't come in were heard discussing that they were going to have the next day off. Parents rang in saying their child was 'tired' - it amounted to about 1/3 of the kids who'd been away for the weekend, and, to be honest, felt like a pretty poor return for the member of staff who'd put their own family life to one side in order to plan and lead the trip.
At that point, though I hated to do it, I felt I had no choice other than to get on the phone and ring the families concerned and explain that while we were prepared to 'authorise' a late arrival in school if they really were so overtired, however, a 7pm return really could not justify a full day's absence from lessons, and in those circumstances, we would not be willing to authorise any absence after the cut off point (11am, I think), unless there was some particular medical circumstance that was a factor. I wasn't the most popular teacher around, and it was one of the sh*ttier days of that academic year, but hey ho. As I say, damned if you do, damned if you don't.
Please don't think I'm implying anything about your own child's circumstances - I really do believe and understand you, and sympathise with how miffed you must be feeling - but I just wanted to explain a bit about the other side of the picture. Would it be possible, perhaps, to ask to speak to the head, one to one? They themselves may not be entirely comfortable with every element of the 'broad brush strokes' approach that someone feels they've had to take here.
Computerised records can be amended - of course they can. However, by the same token schools are not required to 'authorise' absences that they are unwilling to authorise, even if a parental letter has been sent in explaining the reason for non-attendance.