Translation:My younger brother is a sixth-grade student in elementary school.
In the US, public elementary school students usually spend the entire day in one classroom with one teacher. Historically, most public elementary schools taught kindergarten through 6th grade using the one teacher and one classroom format.
In a place like New York City, overcrowding was the main reason they started pushing 6th grade classes into middle school buildings and moving 9th graders into high school. At times when high school dropout rates were high, you had empty classrooms in high school buildings and not enough classrooms in elementary school buildings.
There are so many ways of describing school systems and where one is in them that it is just easier to use the word bank version of questions like this and get on with the rest of the lesson rather than try to wrangle Duo to recognise the terminology used by users of all the different school systems around the world. If you use the word bank version you know what answer Duo is looking for as there is only one possible answer.
This answer is ridiculously wrong from a UK perspective. Perfectly acceptable alternative translations are not accepted. I was marked wrong for putting 'at elementary school' rather than 'in elementary school' - or was it the other way round. Either is perfectly acceptable in English. Primary school, which is the correct UK usage, is given as a possible translation in the hints but is not accepted, only elementary school is allowed which is American usage. Sixth grade is not a UK term either - it's year six, or we would say 'is in year six'. Looking at all the comments over many years this is clearly an unsatisfactory question and should be changed. Or you should allow variations.
"In elementary . . ." is quite common, at least in the middle of Canada. One option that would be common here would be "My younger brother is in elementary grade six." with "school" understood, or even "My little brother is in grade six", with "little' for "younger" and "elementary" and "school" both understood. And of course in the English-speaking world the relative age of a sibling is less an immediate matter of concern, so more common still would, I think, be "My brother is in grade six."
In Canada we would say 'My younger brother is in grade six/ (the) sixth grade.' Sixth grade always implies primary school so it's redundant to say primary/elementary. I belive in the UK this would be 'My younger brother is in sixth form', but I don't know how long primary school typically continues in the UK. The current English answer is too country-specific and also redundant and wordy, but from the Chinese perspective I think I understand why they've written it this way. Chinese students typically speak of being in 'first grade of primary school' and 'first grade of middle school' (the sixth year of state education, I think) and 'first grade of high school' (i.e. what in Canada would be grade 9). It's not a good translation into English ('first year' would be much better) but that's what most people here say.