When it comes to seating arrangements in classes, I rather prefer things to be consistent; if there isn't a set seating chart, then wherever I sit on the first day of class is usually where I sit for the rest of the semester, term, year or what have you. It's a psychological thing; I tend to think better when I stay in the same place every day. Most people respect this, in fact, most people I know are the same way, so it works out and everybody is happy.
In my Biology class, my roomie Beth and I sit in the back on the left side of the room. That is where we have sat since the first day of class and so our unspoken claim has been laid on those spots. This remained unchallenged for a while, until one day we walked in to class to find that three of our classmates -- a boy and two girls -- had unscrupulously taken up residence in our seats. Completely disoriented, we were forced to take seats on the other side of the room. This was not an innocent occurrence either; as soon as we walked in, they met our surprised and slightly affronted expressions with looks of purest hatred, which only intensified the next time we got to class before them and took our proper seats. From then on it has been war; these usurpers exercise every opportunity they get to take what is rightfully ours and upset the balance. It has reached the point where both Beth and I are completely paranoid and race to be there first in order to stake claim on our territory. And heaven help us if we don't, because every time they win, they give us the same glares which speak of nothing but the deepest loathing. I really kinda wish I knew what we did to them, because I'm not kidding about the glares... If looks could kill...
This is really one of those things everybody take for granted until the balance is thrown off. There are certain unspoken rules that should never be broken, and classroom territorial claims are definitely among the things that should not be disregarded. If you want to get by in academia you follow the rules, or forever have your peers wishing you a slow and painful death. Students are territorial.
Don't be a usurper.