Hello, guys. Recently I received my essay and I noticed that I had this error:

I wrote: 'A hundred years ago, nobody had a television set, ...'

Is this OK? Because my teacher suggested that I should write 'A hundred OF years ago'... And of course, I forgot to ask her when she returned it to me, silly me Emotion: wink
I respectfully disagree with your teacher.

A hundred years ago, or Hundreds of years ago... but not A hundred of years ago.
You may want to show her/him this, at the New York Times:

Students: Are you brave enough to let our tutors analyse your pronunciation?
Your teacher is definitely not a native speaker of English.
Teachers: We supply a list of EFL job vacancies