articles (the poor, the old, the unemployed, the teachers (?), the bankers (?)

vor 6 Jahren

 Hello again!

I was trying to explain that we use the definite article "the" with groups of people like the sick, the poor, the old, the unemployed. 
The person I explained this to said that she understands and gave me the example to this rule.
"The teachers work hard."
I understand that it is a mistake to use "the" in this case.. Could you please explain why?

Thank you very much! 