"Where is the classroom for the Japanese class?"
For those of you trying to wrap your head around the grammar, I have a tip: Just like you can ask "whose is it" in English (think of it as "who's", or "belonging to who?") to ask whom does something belong to, in Japanese you can ask something akin to "where's" (belonging to where?) or even "when's" (belonging to when?).
So what you're really asking is: "As for the Japanese lesson, where's (possessive) classroom is it?"
In English, you can only ask whose is it. In Japanese you can also ask about "when's" and "where's".
in English, we think about the sentence in 2 parts:
Where is the (classroom) for (Japanese class)
but, in Japanese, you split it up like this:
(Japanese class) is (which classroom)?
hopefully this helped solve some confusion...
pausing after は gives people time to think, so structuring thoughts this way allows for natural pauses in the sentence in Japanese.
日本語【に・ほん・ご】 の 授業【じゅ・ぎょう】 - Japanese class
は - connects class to rest of sentence
どこの教室【どこ・の・きょう・しつ】 - which classroom (location)
ですか - is it?
授業: the act/period of teaching/learning (le cours, la leçon)
クラス: the organization of students in groups (la classe)
教室: the place where the teaching/learning takes place (la salle de classe/cours)
(*edit: the caveat being that all these French words can obviously also have multiple, overlapping meaning...)
But in this case, arguably either クラス or 授業 could work for "Japanese class", as the room is just as much for the group of students as it is for the act of teaching the subject. I know I've heard クラス used in sentences like this, but to be fair a lot of the Japanese I hear is spoken by bilingual speakers who are influenced by English.
When I see this, I think of this sentence like "As for japanese class, which classroom is it?"Even though どこ doesn't exactly means "which" but rather "where". But I still don't really fully understand the grammar behind it. When I have to write this sentence I try to make "classroom" and "japanese class" as one big topic, behind "は". Like this "日本語の授業の教室はどこですか". But the more I compare it to the correct answer the weirder it sounds.
授業（じゅぎょう）and クラス are mostly interchangeable, but there are differences. 授業 is an actual lesson, so you would use that, e.g., if you were to say you are skipping a class/lesson「授業をさぼる」. While クラス is used to divide people into groups, e.g., a class of 20 students「グループⓐは２０学生たちのクラスです」, a group of people who belongs to class A, and that may be attending a 授業. 教室（きょうしつ）is a classroom.
____がどこですか feels quite strange to me, with using the が particle on the thing you are asking about. I think like I'd normally say it actually is "wrong", however I've done barely any Japanese grammar study in ages to the point that my mind has gotten hazy on the certainty of a lot of things.
I'd be more comfortable using a second は particle like 日本語の授業は教室はどこですか than using that が there.
It just feels strange to shift the focus away from どこですか. I know you can use the が particle in 教室がどこ when it's a subordinate clause (e.g. 教室がどこにあると思っていますか, "Are you thinking 'where is the classroom'"?). However, that's not the case at all for the current sentence where the question isn't in a subordinate clause but is the main clause of the sentence?
If I were to guess, it's because leaving out the last part of the sentence is akin to asking "Where classroom for Japanese class?" as opposed to "Where is the classroom for Japanese class?" In your first sentence, there needs to be a "no" character between the kanji for "Japanese" and "class" (i.e. between nihongo and kyoshitsu); otherwise, they aren't technically connected and the sentence doesn't make sense.
What confuses me is that it marks it incorrect when you put the katakana for "class" (kurasu) instead of the kanji (juugyou). Does the katakana for "class" not mean the same thing as the kanji for "class?" (And yes, I made absolutely sure that I was using it in place of "class" and not "classroom"-- that was my first thought and I triple-checked that I hadn't done something bird-brained!)
クラス vs 授業 - strictly speaking, the former refers the group of people who make up your class, whereas 授業 refers to the lessons/course in an abstract sense, though there is some cross-over in modern usage. In this case though either logically could make sense, and クラスの教室 is commonly used from what I can tell.