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 acceptable although not used that often
If you never use 'ler/lar' with a verb, it will always be correct :)
It is false to use "ler/lar" if subject is not alive.
If subject is alive, it is up to you.
Not very many people actually observe or know about this rule though :) We accept -lAr if the subject is plural almost always