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Use English English except where doing so would cause confusion, in which case use neutral words. If you can't do that, maybe clarify with extra information, (e.g. for "Year 8", you could also add ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47568 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Use English English except where doing so would cause confusion, in which case use neutral words. If you can't do that, maybe clarify with extra information, (e.g. for "Year 8", you could also add in the age of the character, which makes it clear what grade that year represents) or if you absolutely have to, a footnote or have the narrator explain or something. The reason being, since you're going out of your way to set the book in England, and having the main character be English, you want to give it an English flavour, not an American one, otherwise what's the point in setting it in England at all?