When we were wee kids, prepubescent and hairy only on our heads (save for David H.), we took for granted that our elementary education will end after 8 years (in a K~7 school). All the friends you met while tussling around on the gravel filled playground will likely be with you until grade 7, except for the exotic few that become transfer students. We were dynamic back then; I say dynamic because two-faced has a horrible connotation a la Batman. We can like some of our classmates one day, and then disregard them the next, only to be best friends before the week is done. All we know was that we were stuck together until grade 8.
Then comes high school. The sparkle of newness, and what some would suggest where work actually starts kicking in. Freshmen usually have their bouts of friends shuffling as people's interests diverge from lego building and tag on the playground, but typically seniors have a set clique they belong to. The transfer students become less unusual, and they too integrate well with the rest of the school's society. As we sit in our gowns in our grad ceremony, we wonder about those that weren't there with us - the kid in math that moved onto a better high school last year, the girl that had to drop out of school for personal reasons, and the guy who didn't pass enough grade 12 courses to graduate this year. Yet, most people at the valedictory has been there for the whole 5 years.
I guess what I'm getting at is that there are typically a set amount of time you spend at each institution, before you are shuttled off to do (usually) bigger and (hopefully) better things. That breaks down once university starts. There are so many factors that goes into determining how many years your education is going to take up. There are people that I started university with that are going to be gone at the end of next year, and those that will be graduating in what is the typical 4 years, as well as those who will be stuck doing their degrees in 5 or more years, for whatever reason.
All I know is that, a bulk of the people that I started UBC with will be gone before I'm out of here, when I'm out of here. Nothing set in stones, but I'm looking at being UBC grad '13.