Mentally, too, like many of her peers, Keiko is more mature than children 10 years ago, says her teacher Maiko Ikeda. It's time for a change, he says, in how children are sent to middle school at the relatively late age of 12-plus.
The government is now suggesting the nation change to a nine-year system that will combine primary and middle-school curriculums. This would enable staff to introduce middle-school-style teaching of specialist subjects to 10-year-olds.
One ward in Tokyo, Shinagawa, is set to be the first to roll out the new system at the start of the next school year in April.
Ten to 12-year-old girls in particular are experiencing a spurt of mental and physical growth not seen in past generations, says the Shinagawa board.
Two to three years later, both boys and girls reach a great psychological turning point, it said.
Since the 1940s Japan has followed a six-three-three system - six years of elementary school followed by three years of junior high school and an optional further three years at high school.
Shinagawa's nine-year system will allow primary and middle- school pupils to learn in the same school buildings, which will also ease the strain on education boards caused by Japan's declining birthrate.
The phenomenal spurt of maturity in today's Japanese children has been put down by some psychologists and doctors to the increase of meat and dairy protein in the Japanese diet. Girls and boys are also reaching puberty earlier than their parents' generation by about one year.