The levels of education that use "grade" in AmE - 1st grade, 2nd grade, etc. - use "serie" in Brazilian Portuguese. "first grade" = "primeira serie". It's a different phrasing.
In any case, whenever you think your answer should be accepted, but isn't, just click "report a problem" - "my answer should be accepted" under the question and the course contributors can take a look at it.
But there is a context! In brazilian portuguese the word "série" has different meanings: 1: School succession: annual division of the studies in a course (grade or school year). 2. movie or TV show broadcast in parts (see Brazilian series) In the context of the sentence is clear that we are talking about school succession and not about tv show.
Feedback to editors: Grade is only used in N America for a person's school year- this appears as the only form in too many test books confusing too many students of English. In UK, your grade means your score in the test, e.g. A, B, C grade etc. failing to make both forms available leaves a path ripe for confusion. Thus "year" or "class" should also be accepted.
I get it ceaer. I didn't click on "my answer should be accepted" because I wasn't sure that it should be. Sometimes I get frustrated when I don't have a context and have to randomly pick an answer, knowing I have a 50/50 chance of getting it right. If I happen to pick the right one, I have learned nothing. Duolingo tells me I am correct and I still have no idea what I have just written. Now that you explained the school system it makes perfect sense. Thanks for that.