A good part of BtVS in Seasons 1 - 3 was the background of teen angst, the mystique of growing up in the high school world. One of the things I liked about Season 7 was how, in the early part of the season, they had episodes which were nostalgic in tone, with stories set in the new high school. And remember-the Buffy troops ride to their final battle at the high school in, of all things, a yellow school bus!
But thank goodness they moved beyond the high school after Season 3. I wholeheartedly agree that it just doesn't work for a show to stay in high school season after season. I very much regretted the departures of Cordelia and of Angel, as well as losing the library as a meeting place for the Scoobies. I was taken aback at the ending of "Graduation Day Pt. II", I thought the giant snake demon thing was a loser and I was aghast at the way they defeated it, just blowing the high school up. But still, all of that emphasized that there really is discontinuity at the end of high school, no matter how much one loves or hates it.
I've always thought Buffy was remarkably good at transitioning from the high school setting to a new one in the adult world. There was brilliance in all of the four later seasons. But I can't help thinking of the four later seasons differently from the first three. There's a sadness in knowing that time passes and that life moves on.