Firstly, the age of menarche in Western nations has been declining...
Menarcheal decreased over time in Western countries until cohorts born in the mid-20th century. It then stabilised, but limited data are available for recent cohorts. data were collected retrospectively by questionnaire in 2003-10 from 94,170 women who were participating in the Breakthrough Generations Study, aged 16-98 years, born 1908-93 and resident in the UK. menarcheal declined from women born in 1908-19 (mean=13.5 years) to those born in 1945-49 (mean=12.6 years). It was then stable for several birth cohorts, but resumed its downward trend in recent cohorts (mean=12.3 years in 1990-93 cohort). Trends differed between socio-economic groups, but the recent decline was present in each group. In conclusion, menarcheal appears to have decreased again in recent cohorts after a period of stabilization….
Surprisingly, we don't have good recent data - this is the best I could find. There's no evidence of a similar shift in males.
Secondly, in athletics, we know a maturational advantage of a few months can make a big difference in competitive events.
High school education is a competitive event. So, if girls are maturing earlier, how does this impact their academic performance relative to boys? Does this explain why our middle and senior high school honor role events are all female? Do boys get discouraged because they can't compete?
Seems like an interesting question. I'm thinking an 'all boy' school might be a good idea for our middle son.