"Many boys are students."
Translation:Multi pueri sunt discipuli.
From the Duolingo Latin 'Introduction' notes:
The nominative case is the form of a noun you will find a dictionary. It is used for the subject of a sentences and for predicates following a form of "to be".
The predicate is the second part of a sentence following the "X is Y" pattern.
- I am a man. -> a man
- These women are engineers. -> engineers
So here we use discipuli since we are using of sunt, a form of the verb esse ('to be').
You can see all the notes given by Duolingo here (Introduction is the first one): https://duome.eu/tips/en/la
The section Predicate Noun or Adjective at this site may help give a better explanation: http://dcc.dickinson.edu/grammar/latin/agreement-nouns
I view it as being in the nominative with forms of esse (here sunt) because it describes rather than is something that an action is being performed on. The second link should expand on this.