Hmmm...
Sounds quite arbitrary.
FWIW, 'got' is actively discouraged in American English except in its proper function as the simple past of 'get.'
Our teachers think it sounds schoolyardish.
And one hardly hears 'jumper' at all. We'd use it mostly, if at all, for a pullover dress.
For a sweater it calls attention to itself as British.
Then again, we could also start quibbling about the meaning of
pusero, which I consider a broader concept than sweater, more like what Americans would variously call a blouse or top or shirt. I'm used to hearing
villapusero for sweater. So possibly the issue is not what word he uses for sweater but how he translates or understands
pusero.
At that age I suspect there is more to lose than to gain in making an issue of it with the teacher. She'll be out of the picture next year, your son isn't getting
bad marks, and he's too young for these dings to make a lasting difference. I would guess it is an attitudinal thing on his teacher's part, though there are several different attitudes it could be expressing. Among them would be insecurity and a chip on the shoulder. I would tell him to do it her way in class even though you both know better for real life. In Finland,
that's a useful life lesson in its own right.