" - " = "are"?
Yep. Usually is/are is just omitted and that's it. But if subject and predicate are both nouns, we need to put dash between them
Singular or plural, it seems to me.
You can also say "moyi druzi ye studentamy" (ye=are), but I'm not sure Duolingo has coded this as a right variant.
Can it also mean my student friends?
Ex in a sentence = my student friends are there = modi druzi - studenty tam ? (Students here as in adjective to noun friends)