A question of trust

Why do I trust one student to take an exam unsupervised while I require another to sit in my office and write while I watch? 

It's a matter of trust. There's nothing like exam season to inspire a careful parsing of the parameters of trust.

One student needs to take the exam early so she can leave town for a college-sponsored activity; another dashes into my office and begs to be allowed to take the exam even though he overslept and missed the exam period.

One student has taken several of my classes and always demonstrates responsibility, character, and perseverance; the other frequently skips class, sits in the back, rarely participates, and provides no clear evidence of virtue.

One student is taking an exam for which notes, textbooks, and other students would not be helpful, so it would be difficult to gain points by consulting sources; the other is taking an exam requiring identification of authors and titles of works and specific examples of concepts from certain works, so having access to sources would give the student an unfair advantage.

Which one would you trust?

And after you've figure that one out, please let me know what to do about the student who left the classroom in the middle of the exam to use the rest room and didn't come back for more than 10 minutes...and then wrote about a section of a work that we did not read for class. Who will proctor my exam if I have to start following students into the bathroom?