The system of Night Raven College was explained explicitly by Toboso Yana with the release of the Book 7, Chapter 13 finale:
“Twst follows a European/American-style system, so compulsory education is until the age of 16. NRC is considered a ‘high school/technical college,’ so everyone enters at 16.
NRC is essentially a technical school. Although there is what is uniformly called a middle school, the systems likely differ depending on a person’s country of origin, and there are also a fair number of students who are homeschooled.”
This is also being reflected within the game through characters referencing “elementary school” and “middle school,” using the English-language words, not Japanese.
They also refer to NRC as a high school, using the English word for high school and the Japanese word for “high school student,” possibly due to grammar rules making “high school student” more difficult to adapt.
While NRC is “Night Raven College,” in the UK, “college” is a two-year institution where many students go from the age of 16 in order to prepare for university.
It is different from the American-English use of the word “college,” and Book 7 begins with the 3rd-years discussing whether or not they intend to move on to university after NRC.
As Yana has explained, NRC does not seem to be following one Western system or another, instead having picked and chosen different elements from different systems and combined them: a college (referred to as a high school) that they begin at age 16 to prepare for the future (re: the UK) and lasts 4 years (re: the US), beginning in September (re: UK/US).
And this is actually something that the prefect comments on in the novel: it is possible that everything being so different from Japan (where high school starts at age 15, in April, lasting three years) is to contribute to the double culture-shock being suffered by the main character, who is not only in an unknown world but a world that seems heavily influenced by non-Japanese countries in everything from the school system and the food to the shapes of the buildings:
“He’s heard before that there are schools in foreign countries that begin in autumn. This school’s system seems to be closer to those of Western countries than to those of Japan; though, he had already assumed as much, based on the shapes of the buildings.”
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“Yuuya had heard of high schools that last for four years even back in his own world, in countries overseas.”
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“Thinking about it, Yuuya realizes that he can’t recall having ever seen Japanese food or sweets since coming to Twisted Wonderland. The snacks in the cafeteria and the school store have always seemed to be more in line with what he imagines American and British cuisine to be like. Just thinking about familiar flavors has Yuuya craving them, but if an-bread is being sold here, then Japanese foods must exist after all.”
This means that the EN-server main character might not be experiencing the same degree of culture shock as the JP-server main character, but Aniplex USA maybe didn’t have the ability to change parts of the game like the time of year that school begins, how long it lasts and what foreign languages are used in it in order to get the same general atmosphere across, resulting in a more familiar situation for EN-server prefects than is being experienced by JP-server players.
Twisted Wonderland Reference Note 