Why did the students post their “twelve theses” in the entrance hall of the university?

to show their opposition to Nazi policies
to show their support for a Jewish teacher who was about to be fired
to demand reforms to university tuition policies
to call for the expulsion of all non-German professors

To call for the expulsion of all non-German professors.

Explanation:

"The Twelve Theses" was a text published by the German Student Union. These books called for the elimination of all "Jewish aspects" of culture from German literature. Among many other requests, it asked for a ban on publishing works by Jewish authors in Germany, and a ban on German translations of foreign books. It also called for the expulsion of non-German professors.

there are many answers to this question, such as a diplomat, ambassador, soldier, united nations employee, cia, etc.