Translation:Why is there nobody in the classroom?
"人人" is better translated in most contexts as "everyone", though it could be thought of as "people" meaning "all people". It wouldn't fit in this sentence at all, as far as I know.
Duo's sentence could indeed be translated as "why are there no people in the classroom".
Couldn't the translation also be "Why is the classroom empty?" (empty in the sense there's nobody)
There is a long way to go for this Chinese course. Many, many sentences only accept a few English translations when there are other valid ways to say it.
Agreed. Keep reporting and this course will slowly build up. It's frustrating at times, although they have already done a pretty impressive job, and getting better by the day. They have already accepted dozens of my translations, so just keep reporting! :-)
Agreed. My placement test didn't accurately assess my level of experience because it kept calling me out for having slightly different wording in my translations, which should have been accepted.
This course is still in beta so lots of translations are missing, but even for the mature courses without that problem I found the placement tests to vary widely and not be very useful.
"why is there no-one in the classroom" was not allowed but should be
For me the most natural way to say this in English is "Why isn't there anybody in the classroom?" but that's not yet accepted.
It's widely held that when you have a plural noun or noun phrase with "there", you have to have a plural verb, "are": "there are people"; "why are there no people".
Some linguists/grammarians would say "is" is okay, based on common use, and many (probably most) educated speakers often use the contraction "there's" with a plural even if they'd always say "there are" when not using a contraction.
Duo's probably not going to give you any leeway, though.