"This is my friend in the class next door."
"クラスの友だち" This was difficult to understand in one go.. I think it means my friend from the class (next door)
Yes, となりのクラスの友だち literally means "friend of class of neighbour." The の particle can be used as a possessive (あなたの本, "your book") which is usually the first thing people learn about it, but it's also used to link nouns together, as is the case here; rather than "my friend owned by the class" クラスの友だち means "my friend from the class."
となり is slightly different in that it's often classified as a "no-adjective;" if you use dictionaries like jisho.org you'll probably have heard of those. It works pretty much the same way, though: となり means "neighbour," となりの means "next door."
I chose "kyoujitsu", instead of "kurasu", and Duo said it was incorrect. Is this really wrong to use or should I report it?
FMy first instinct was to put これ rather than こちら, which in my head I associated with directions and places rather than objects or people. Is it rude to refer to a person in this way?
This "no" sentence structure is very friendly to Chinese native speakers since we have the similar "de" .