"But the school knew she wasn't going on the trip. How was she allowed to get changed out of her uniform into her own clothes without anyone noticing?"
Perhaps the parents should 1st consider how she was allowed to stash civvies in her school bag and leave home with added bulk/weight in bag. They are fast to point a finger, but appear to have a blind spot with regards to their own ability to be hoodwinked by their daughter.
If they did not do so, the school should have done a headcount. I'm in TEFL. Our standards, particularly those of decades ago which was the last time I put myself through the hell of taking small children on a trip, tend to be lower than state schools, not least cos we don't/didn't have OFSTED ready to pounce on us. But even I managed to realise the need for and use frequent headcount at every "we are moving off somewhere" point. Kids can be sneaky little buggers at worst, and clueless when distracted at best. They can have "cunning plans" that have the capacity for awful outcomes. You need to count them to make sure you have the right number of them at all times. And look at faces. We almost left one place with a kid from another organisation's group, while one of ours was in a loo having decided to just wander off for a pee without telling anybody. That was my last trip. I don't want that kind of responsibility for other people's children. Especially the kids who haven't benefited from the strongest of parental boundaries, with parents who look everywhere except the mirror when deciding who to blame for their child's latest high jinks. I take my my hat off to any teacher willing to take kids away for the day, let alone overnight. Personally I'd rather stick pins in my eyes than ever do that again.