Translation:Öğrenciler derse katılmalı.
Is it wrong to say: Öğrenciler derse katılmalılar?
I know that Turks generally don't double the "lar" between subject and verb, but I have seen it before in Duolingo lessons.
It is false to use "ler/lar" if subject is not alive.
If subject is alive, it is up to you.