"I am a fourth grader."
It's really not being America-centric here. The Japanese school system has 3 main levels (and university). 小学校 is 6 years and 中学校 and 高校 are both 3 years each. They label each grade with which year of school you're in. In this case, we have fourth grade (although, I think year 4 would be an ok, Harry Potter style translation). It's just how the grades are written. It would be America-centric if 中学校一年生 were called 7th graders or if 高校一年生 were called freshmen. Duo's way is a rather literal translation that frankly works well. I don't know what sort of school system you have, but if you demanded that your grade system be translated into Japanese, it probably won't make sense to convert to the Japanese school system style since they're not the same.
I think this is more of a Duo-glitch than an America-centric issue. I'm American, and it didn't even remotely occur to me to add elementary school to my answer - which was marked wrong.
I don't think anyone would actually say "I'm a fourth year elementary (or grade) school student", as the elementary school part would be evident from their age. We would just say "I'm in fourth grade" or "I'm a fourth grader".
Just report it!
No. Let's think about the literal translation of "小学四年生": it's "a fourth-year elementary school student". Would "a fourth-year student elementary school" be corret? No. "四年生小学" sounds exactly like this last sentence. "Sei" is always in the end because it individually means "student" so the adjective ("fourth-year") always comes before it.
I'm not sure, but "四年小学生" may also be right. However, if you think about it, it would sound like the elementary school is in the fourth grade instead of the student (which is not possible because "sei" is always referring to people in Japanese).
An adjective always comes before what it is trying to qualify, right? So "生" = student; and "四年生" = "fourth-year student" (the "fourth-year" indicates in what grade is the student); and "小学四年生" = "fourth-year elementary schoool student" (the "elementary school" indicates what type of "fourth-year" is that i.e. it distinguishes it from a fourth-year university student).
If you wanted to specify the subject you could use a particle (私は小学4年生です - watashi wa shougaku yon’nensei desu), but otherwise it's just one of those phrases that doesn't require particles, like 東京出身 (Tokyo shusshin) or 海外旅行 (kaigai ryokou). It feels like there should be a の in between the nouns, but there isn't.
I really have to think about my answers here as we don't use "grades" in England & so I have to kinda guess which grades are in which schools until I learn it more thoroughly. Are which grades in which school the same in Japan as other countries like the U.S? Not that I know either of them, I'm just curious :)
Japanese children start 小学校 (shougakkou) at age 6 as an １年生 (ichinensei) and finish at age 12 as a ６年生 (rokunensei).
In the US children usually start elementary school at about 5 when they enter kindergarten, then continue from 1st to 6th grade, though some elementary schools only go up through the 5th grade.
Japanese children attend 中学校 (chuugakkou) for three years from ages 12-15 as an １年生, ２年生, and ３年生.
In the US, middle schools or junior high schools are usually 7th-8th grade, 6th-8th grade, or 7th-9th grade.
高校 (koukou) in Japan is from ages 15-18, and they once again cycle through as an １年生, ２年生, and ３年生.
In the US, high schools are usually 9th-12th grade or 10th-12th grade, with students graduating at about 18 years old.
Japanese is a very contextual language. For most sentences like this, you'll know from the conversation who it refers to. Japanese speakers rarely say 私は unless it is unclear that they are about to talk about themselves or when they want to emphasize that something is their opinion. Honestly, the most frequent place I hear it is on Japanese tv, where there are usually a lot of questions about personal opinions or experiences. I recommend not including 「私は」 in your answers unless you're using the bubble answers.
That's fine if the Japanese school system holds no personal interest for you, but there are plenty of people here who would like to do things like study abroad or have a job in Japan (and one of the most popular jobs for native English speakers is teaching English, for which it's good to know about the school culture). It's a bit much to say that "no one needs this stuff." If you don't plan to talk to anyone with school-grade kids, kids themselves, or teachers, then it's easily skippable I suppose.
If I recall correctly, this is under the school topic. It's a good way to combine culture with some numbers in Japanese that are more consistent than other counters.