For what it's worth, "freshman" is American, and the related British/Australian/NZ term "fresher" only applies to first year students in their first weeks at university - not at schools.
I'm glad you didn't use the word. It would seem really out of place.
In England, we just said "6th year" (cf Harry Potter), "lower sixth" or "Year 12", depending on how old fashioned the school was.
And "first-grader" is something completely different, those are six years old, not sixteen, so that was completely out. Plenty of traps and completely inappropriate translations as well.
I ended up using "first-year", "second-year" and "third-year", and "firstie" for when older students were looking down on them.
I'm glad you didn't use the word. It would seem really out of place.
In England, we just said "6th year" (cf Harry Potter), "lower sixth" or "Year 12", depending on how old fashioned the school was.