why 'Alle Studenten' should be 'all the students', not 'all students'?
Both are valid (as well as "all of the students").
I guess because "all the students" is more natural in English. It's odd though, because duolingo often accepts strange, non-natural literal translations.
@duoluo @BenUserName : "all of the students" is what is natural to me.
"All students" is accepted, too.
Reminds me of "Hearts in Atlantis" by Stephen King... Great read.
why doesn't the second verb (gegen) go to the end?
It's not a verb, it's a preposition.