Chris at 2011-05-27 11:17:25:
The downward trend is a combination of all things your mentioning, but I believe the biggest reason is that no one is really aiming at that age range. All the movies that come out try and hit that wide spectrum from little kids to adults without caring about the middle section. All the superhero movies want to sell toys and be 'adult' for the mature audiences.
There haven't been any high school dramas or comedies that really hits that demo. The last one was probably American Pie. Not to mention, we are sorely missing a John Hughes. There just aren't high school movies that they can connect to.
If I were a studio, I would try and hedge my bets with a wide range of genres/demos and still continue to push action / adventure titles to foreign markets.