Amber is behind because A) she lost a year in elementary school due to the situation recounted in "Amber and West" (a sloppy bonus comic that all $1+ patrons have access to) and B) in Amathae, attending secondary school is not compulsory, so Amber was able to finagle leaving school before graduation (at least, until she ran away in chapter 2 and got in big trouble).

Also, Merry Christmas, everyone!!! Thank you for being the best readers on the internet!!!<3