Which grammatical case would the answer to this question be?
When "o" means "about" (so mostly), it takes Locative. Ta lekcja jest o poezji. (This lesson is about poetry).
o co chodzi ta lekcja?
You'll need w+locative. O co chodzi w tej lekcji?
Would "O czym to ta lekcja?" have been accepted?
To only works with nouns on both sides.
OK, thank you
It's "czym" here. It's the Locative form of "co", meaning "what".
"czymś" is the Locative form of "coś", meaning "something".