Reported for not accepting "my brother is in 3rd grade." The note says you need an article before 3rd grade. As a native English speaker, I'd say it's more common NOT to use the article.
Agreed. I'd report the "correct" translation as wrong too. It should be accepted but not the suggested translation.
So does that mean that the words that describe the place that is in the prepositional case also take the prepositional case?
For instance, if something is going into a red room, then красный would take a sort of prepositional case?
Exactly. To put it the way textbooks do, the adjective agrees with the noun.
Your example is incorrect though, because if you're talking about movement into, then в takes accusative case. So "into a red room" is "в красную комнату" while "in a red room" is "в красной комнате".
"В" can describe either location (in a place) or movement (into a place). If you're talking about location, it takes prepositional case (actually locative case). If you're talking about movement, it takes accusative case.
As a non-American, I think that terms like the "nth grade" should be replaceable by "grade n" or "year n", as is commonly said.