http://www.esri.go.jp/jp/archive/e_dis/ ... dis162.pdf is my source:
For example, primary schools in Japan accept entering students on April 1 and children who are age 6 by that day enter primary schools for that year. Thus those who are born in March (including April 1.) enter primary schools at an early age 6, while those who are born in April enter primary schools at almost age 7.
(The comment about "before the school starting day" is due to a quirk in Japanese age law - your age is incremented the day before your birthday, according to that article.)The Japanese school system is similar to that of the US. Compulsory education consists of 6 years of primary school and 3 years of junior high school. After junior high school, students can choose to attend 3 years of high school or 5 years of technical college (Kosen). After high school, the choices are 2 years of junior college or 4 years of college. There are graduate schools for advanced degrees afterward. For an illustration, see Figure 1. The school system is legally defined in the School Education Law (SEL) enacted in 1947. SEL article 22 requires parents to send their children to primary schools once their children will turn age six before the school starting day, which is April 1. Article 23 allows a delay in school entry due to children’s illness or underdevelopment, but this exception is rarely applied.
This creates a problem with three characters, and also requires Hisao to have been born 1989-04-02 or later.
Lilly turned 18 on 2007-02-07 (and this is confirmed by Hanako in Lilly's route). Working backwards, 1995-02-07, she'd be 6, meaning she'd enter primary school in 1995, junior high school in 2001, high school in 2004, and her third year of high school in 2006. So, she missed a year somewhere. Where did she miss it? I doubt she'd fall under article 23, if it's rarely applied (unless her parents pulled strings in high places to get her start of school delayed for some reason or another).
Rin turned 18 on 2007-03-13 according to the website. Same thing, her third year was 2006 in that case, or she's 17 (it wasn't stated that she was 18). This one is easier to explain, though - she may well have fallen under article 23, as she could quite easily appear developmentally delayed.
Emi turned 19 on 2007-03-14 (she says she's a year older than Hisao, who's presumably 18 (Hisao implies that he's 18 in Lilly's route, although he doesn't outright state it), although she could've been mistaken). She also says that she was held back one year, but for her to be 19... yep, her third year would still be 2006. Was she held back another year, or is she actually 18?