When it's a "field trip" for minor children, the chaperones have some authority. I would guess a few teachers or parents went along--I don't know, but that'd be reasonable. Picture trying to restrain some frisky young thing--who happens to be an adult. You could do...what? Express some disapproval? Try to dissuade? Frankly, you can have a little moral suasion...that's all.
What has struck me about all this is that if Natalee had been a chubbly little dump with bad skin...nobody'd give a hoot. There's a lot of mourning the loss of a hottie here.
She'd still be alive.
Those guys wouldn't have given her a second glance.