***** NOT A TEACHER *****
The teachers have already given you the answer.
I just thought that you would be interested in this:
A few (very few!) people prefer to use "so" in negative sentences:
Rome is not nearly so beautiful as you.
***
Those "few" people explain it this way:
English speakers hear "as ... as" so often that it is better to use "so ...as" for
negative sentences because it will make a bigger impression. Those "few"
people feel that "You are not so nice as my last teacher" will make a stronger
impression than "You are not as nice as my last teacher."