Think about it.  TV shows that revolved around high school characters were a hit.  But when the characters graduate from high school and enter a new stage in their lives (that is, college) ratings begin to drop and eventually the shows are canceled.  Here are some examples.

Saved by the Bell.  I loved Saved by the Bell.  Zack Morris was a genius.  There were six total seasons of the show before the original series ended.  And then came the spin-off:  Saved by the Bell, The College Years.  The show included 4 out of the 6 original cast members, and a few new faces.  The spin-off only lasted one season before they officially ended the entire storyline with a movie. 

Dawson's Creek.  Dawson's Creek is the original teenage angst drama.  The show lasted four seasons before the main characters had to graduate high school and go on their separate ways.  The show only lasted two more seasons with the characters at the college age.  But the ratings continually dropped by the fifth season. 

One Tree Hill.  This was one of my favorite shows.  One Tree Hill did things a little differently than the other dramas.  They decided to skip the four years of college and fast forward to what happens after college for the characters.  This show is still doing okay, currently in their sixth season.  The ratings are definitely not what they used to be, but they are still attracting regular viewers every week.

The O.C.  The O.C started off as such a hit!  And then what happened?  Viewers began losing interest when Marissa died after high school graduation.  And then things went sour as the main characters coped with their close friend's loss...and the transition into life after high school.  The short-lived drama lasted only four seasons. 

Gossip Girl has yet to evolve to college level yet.  But I'm thinking that if and when they do, it will fail like the others have as well. 

If you think about it, have there been any dramas that circled around college life?  No, right?  The only one I can think of is Greek.  Which isn't really a real hour-episode drama that comes on basic television.  Maybe it's because nothing compares to teenage drama in high school.  Perhaps college students are just so mature....yeah right.