You have to say your child has only experienced 3 years of schooling, if it's a good school they will listen, year group tally is right but in Scotland we start a year later, they might not know that. I'm a teacher and it's really important otherwise your child may find themself treated as sen. Point out to them your parent's rights, also it's supposed to be an international school so really should be flexible. It's probably ignorance of the Scottish system, good luck but stick to your guns!