Is it somehow correlated with the QUALITY of immigrants that have been coming to the USA and attending our public schools?
I reckon so.
That sounds like a reasonable factor to include in the overall picture.
You would also have to consider the internet, smartphones and gaming too IMO. I think the elasticity of the mind weakens when a person focuses intensely for hours and hours at a time on the limited experiences those machines offer. Especially when the fascination with it begins in childhood.