"There have been..." is a very common English construction: "There have been times when I thought I would never learn this language, but now I can speak it fluently." "There have been changes that I never could have imagined." "There have been lots of people who were special to me, but no one as special as you are." I think your English teacher may have been speaking about something relevant to a context I don't see here.

The construction is "there + to be". "To be" is conjugated according to the tense. e.g. There is - present simple singular There were - past simple plural There have been - present perfect plural There should be / There could be / There must have been, etc.

"There have been" is correct because "demands" is plural. If the sentence used "a demand" instead then it should read, "There has been a demand.." You can often check your accuracy with subject-verb relationships by rearranging the sentence to see what sounds right. For example, you would say, "Demands have been made." You would not say, "Demands has been made." Hope this helps.