Chinese grammar usually. follows STPVO.
This sentence is still proper, but I could, and usually would, place今年after谁.
With a declarative sentence i would say you are correct, but in this case your advice is absolutely wrong. 誰今年教你們的班 sounds stilted.
Since it seems we have here a disagreement between native speakers, it woiuld seem to follow that Duolingo should accept either as permissible.
Am I the only one who would say 今年你们班谁教?
I could see someone say that IRL.