September is a wake-up call for high-schoolers, as summer days at the park make way for integral calculus. It’s even more jarring for bond traders, whose number-crunching around the autumnal equinox is more stressful than the fall semester’s first 12th-grade exam. That’s because the ninth month of the year is the worst month for longer-dated bonds: In the last decade, global government bonds with maturities over 10 years suffered a median loss of 2% in September, according to data compiled by Bloomberg. [...]