AOG - “Is it the education system, is it the teachers, is it their parents, or just the teenagers themselves?”
It's not the teenagers. Of that, I am sure. You'll always get some individuals who, for whatever reason, are going to be intellectually below the norm – this is to be expected and is true today just as it has been true in every generation that has been before. However, there is no conceivable way that in a few brief years we have given birth to a entire generation of youths that have taken an evolutionary step backwards and become significantly less intelligent than their forebears.
I don't think it's the teachers either. They work within a framework not renowned for its flexibility. The methods of teaching and the specifics of what they teach are all dictated to them from officials in government.
The problem is the eduction system as a whole, the parents and the society within which they are raised.