"Which grade are the children in?"
Translation:¿En qué grado están los niños?
I had thought that when there is a limited number choices to use cuál, but looks like I was wrong: https://www.realfastspanish.com/vocabulary/que-vs-cual Apparently when a noun follows, you always use "qué". Its only when a verb follow (but not ser) that the limited versus unlimited choices plays a role. If the verb ser follows, then "qué" then the meaning is being asked, and if cuál, then one is asking about the thing itself. The link above has a much better explanation.
There are lots and lots of examples of using "cuál" immediately before nouns in Duolingo sentences. I think the "qué-only" rule is the traditional one, but levels of adherence to it vary by region and the noun concerned.
I've looked up several of the comparisons on Google NGrams; "qué" is always more common, but how much more common varies a lot. For "grado," "cuál", indeed, hardly registers.
I'd be curious for the perspectives of native Spanish speakers.
Fantastic link I finally understand the reason for "Qué..." in this answer! hank you
Ignore my previous responses! I now know why it is "Qué" and not "Cuál" in this example. If you IMMEDIATELY follow "what" or "which" with a noun in the English translation, then you ALWAYS use Qué. So, e.g..... Qué grado estás, Qué libro lees, Qué coche prefieres. I know it flies in the face of the rule we are often taught of What versus which, but hey, no one ever said this was going to be easy lol
In previous exercises Duo has used 'En cual nivel estas?' so i don't see the distinction.
The fact Duolingo used it is unrelated to whether Spanish speakers will necessarily accept it. In every such thread in the English from Spanish course, you'll see people complaining that "cuál" used like this isn't grammatical. This isn't the only opinion there is, but it is an opinion that exists. Nobody will object to "qué."
I'm told by native speakers that "En qué..." is better in this context. Although my Spanish teacher says that "Qué" is used when you need a definition in your answer and "Cuál" when you want to have a choice between things as the answer. So to my mind cuál would be better, but according to natives, not!
I think the distinction is whether it's qué/cuál + noun vs qué/cuál es + something. In the former "qué" is the general choice with "cuál" being less than universal. What your teacher said applies to the second case.