Slower to mature
Boys develop non-cognitive skills slower than girls. It is not the school system that lets boys down, but their biological development.
Education is much less selective than it used to be. Students are not removed from the school system in the same numbers as they used to be; there are also legal mechanisms in place to prevent children from not completing secondary education. This has led to more disengaged male students taking exams and lowering male performance figures. Males academic performance has not deteriorated in recent years. It is just that the sample size has been broadened to include every male in the school system instead of only those that were engaged.
In the past, many male students who would have fared poorly in exams or were disengaged with school would have dropped out. Their results would not have been included in national statistics and their absence would have bolstered statistics on male academic performance. These disengaged and poorly performing males are now included in the results pool. The phenomenon of the gender educational performance gap was likely always present but not visible until recently. If we can deduce that it is not a modern phenomenon, then we can dispel many of the myths that suggest the modern school system is to blame for male underperformance. If it has always been there then it clearly is not linked to a decrease of competitiveness in the school environment or a shift to more explorative learning styles. More likely, it is a societal issue or cognitive issue.
While it may be true that more disengaged male learners are now being included in national statistics, it doesn't explain away all of the issues associated with the gender education gap. Male enrollment in higher education, an indicator of academic performance, is alarmingly low. At some universities, female students now outnumber male students by six to one.  Gender proportions on college campuses across the US are now weighted 56-44 in favour of female students. This still points to deep-rooted issues in the modern school system that make it difficult for males to excel and get ahead. The whole phenomenon of deteriorating male academic performance cannot be explained away by statistical anomalies. There is still evidence of an underlying bias against male learners.
[P1] Male academic performance is not deteriorating. [P2] Therefore we can dispel many of the myths surrounding male underperformance and a biased school system.
[Rejecting P1] University admission show that male academic performance is deteriorating.