I don't think they are getting "stupider". personally i think they are trying to learn to much of everything.
I went to high school in the early 90's. i had 8 classes with having Math, Science, English and History every year and every day. I then had 4 choices i could take (but some classes were mandatory also had to have 3 years of PE).
My wife's younger sister is 15 and in high school. she has 14 classes! A and B days. its rather insane how many things she has to learn.
Also she does not have to take PE at all and only 2 years of math and history.
So instead of being really good in the core classes (three R's for us older people) they are meh at all.
note!: i feel like shit and on drugs (legal) so if thats not clear sorry heh. but sinus medicine screws me up LOL