Why is research on "conformity" against the law?

Is it?

According to my professor, to assign students to write essays over the
topic is illegal, but myself along with some of my class mates can't
find anything over it.

For what course name/subject do you have this professor? That would be a clue.

If s/he is a Social Psychology prof then they are probably testing the
class to separate the conformists who follows such a directive from
the independent, critical thinking, explorers such as yourself.
Else it is a word that only seems to sound like kon forma tee.

What country are you in?  Unless you are in an authoritarian regime,
the prof. must be smoking something.  If you are in an authoritarian
regime, it is foolish to have posted this question, as it could get
you in trouble.

Whose law?  Ask him to quote chapter and verse.  Oh, that should be
Section and Article, but it raises the point:  does the school/college
have an orientation that would make the subject taboo?