Why go on with the field trip at all? For all the talk about "you feel the pressure to be that authority", wouldn't the best move have been to abandon the trip and side with the student?
Not sure that would achieve much, except maybe make her feel (unfairly) guilty and the rest of the class potentially resentful (despite this not being her fault).
But sending her away makes no sense either. Why not ask her to wait outside, ensure the rest of the class finds their way, then attempt to find somebody with suitable authority to allow her in?
There's no point arguing with a security guard past a certain point; they simply do not have authority you're seeking.
However, even if you spend the entire session trying to find somebody and don't succeed, at least the student will respect and appreciate your effort and won't feel alone in her plight.
The story was abbreviated: I talked to several people to figure out what was going on before they insisted we were blocking the entrance. By then she had decided it was not worth the trouble and wanted to leave.
