For example, if someone in school got in trouble and he got sent to the (female) counselor's room, and there is no person but the counselor and the boy in the room, would he be sinning in this case because he was alone with a woman? What if the door was open and other people weren't far away, but still out of sight?
Because of the hadeeth:
قال نبي الاسلام عليه الصلاة والسلام : " ما خلا رجل بامرأة إلا كان الشيطان ثالثهما "
Jazakum Allah khayran,
Hadith is to avoid to be alone with opposite gender as precaution but if you don't have your will or option to avoid, you'll not be held liable but in such situation you have to try your best to protect your chastity and remain modest and seek Allah's protection and help to save from evil.
As in story of prophet Joseph (Yousuf) in Quran chapter 12, verse 23-27, he got in similar situation.
I think that the hadith entails a situation in which they are more likely to mingle with one another, moreover if that is the case it could also be that one or both of the people involved may resist and not listen to Shaytan. The hadith doesn't say that illicit relatioships will certainly occur, it just states that shaytan will be trying to allure one or both of them into lust, but not succeed.
EDIT: It could be also that a girl may be reluctant to be with a boy in acertain moment (a beautiful girl and a nerd, classical example) and in that case shaytan cannot do that much...
EDIT 2: Many times I've been in these kind of situations, and trust me, even when I wanted, nothing really happened. I live in an environment full of fitnah, but what I personally see is that it is hard even to fall in that specific fitnah EVEN if it is willed. Many claim that it's easy to get a girlfriend and bla blah, but in my case it would be the opposite if I wanted.
