Safeguarding doesn't equal 100% perfection.
If the staff took a register, had a look, did a head count or any other boarding check then they did check.
No safeguarding procedure has a list if every possible nefarious act that has to be remedied prior to any activity being arranged. That would be ludicrous.
However, this school and these staff will now be held to a higher level of accountability and their SOP for trips will henceforth include checks more closely linked with the security services - mirror check under every seat and a sweep of the baggage area, perhaps.
Yes, they missed her sneaking off to get changed and sneaking onto the bus, but neither of those are normal/forseeable happenings - though they will be now in this LA!
And they did notice, 15 minutes away from school. She was returned to her parents and no harm was done.
With the additional info it become more obvious why she was so determined to go... and I am still full of sneaking admiration for her whilst irritated that her parents are trying to make a mountain out of a molehill.