A more likely cause for these results is that there has been an effort in the last decades to dumb down education at every level.

Not necessarily in order to lower the average level of education but to battle perceived unfairness or racism within education.

The underlying assumption is that if you find that some minority or group is not represented equally in any area of education then it is a sign that there must be institutionalized racism involved.

This process is still ongoing to this day, currently everyone is focusing on STEM fields and asking the question: Why do so few women choose to enter these fields?

You can bet that the quality of education you are going to get in 20 years in these fields will be worse in order to attract women to fields that they are generally less interested in. (if Mathematics is the reason for that then they will simply remove it or reduce it significantly)

Edit: It is by the way more likely that changes in the education system (of which there have been plenty) cause different results in education than attributing it to something like contraception.