We have a generation now that is plugged in to blocking us out. They claim to have a right to do this too. I am not prepared to teach children who are apparently listening to me in one ear and the Scissor Sisters in the other.
I admit to liking the aforementioned band but not as a soundtrack to teaching. Enforcing this rule has become stressful and my college is not prepared to make a stand against this trend.
I would agree that this has become a problem that some schools and colleges have been slow to deal with. MP3 players can go on playing all day and are easily concealed. In my experience, as soon as I ask a student to remove their headphones they surreptitiously replace at least one earpiece, often with the head turned away from me so that I can't see.
Has their attainment got worse? I don't know. I do find though that during periods when students are working independently music of any type helps them to focus. But I do insist the volume must be kept low and they must remove both earpieces when I am addressing them. It is a middle ground but it works!