Student or Learner
I have noticed that people use both those options as their statuses in a messenger.
To me, 'In Johannesburg' is correct while 'At Johannesburg' isn't.
At least Murphy's book can back me up on that.
So why use 'at' ? Is that because of lack of education or there is another reason?
[not a teacher]
I can see using "at" when the city name represents something other than the city proper, say Johannesburg College or Johannesburg airport.