No, the school is not totally at fault. Saying that absolves the child from any blame whatsoever. I cannot imagine even thinking about this at that age, because I knew it was totally wrong (and that my parents would lose the plot with me). She planned it - took the clothes into school. Her parents seem not to have spotted that so they are quick to shout about other people's failure to notice something, not so loud about their own oversight. Yes, there needs to be a look at exactly how the school does this - is it a policy failure or a failing on this one occasion? Clearly there was an issue somewhere - was she hiding under people's feet for example? Did they do a seatbelt check? But if you bay for blood from the school, all that will happen is teachers will say "I'm not doing another trip. Spend a week away from my own kids, not sleeping properly, and end up being hauled over the coals when a child does something they absolutely know they should not be doing? No thanks."