http://www.theatlantic.com/entertainment/archive/2012/09/how-to-tell-that-a-teen-tv-show-has-exceeded-its-expiration-date/262397/

From "Glee" to "Gossip Girl."

i think Veronica Mars is a bad example - they didn't cancel the show because she graduated high school, it was canceled because the wet blankets at the CW were too stupid to know what to do with a good show that didn't fit their 12-17 year old female demographic that they are currently aiming for at the expense of all others.

- on the other side of this, look at Degrassi (which I don't watch, but its obvious that it must be still doing well) - they have managed to keep a high-school themed show alive after what 11-12 seasons? they introduced new characters each season seamlessly and kept the main characters.