However, once we move into the field of adult education/training, then teachers are very often just facilitators. Except in the specialist field of the subject, they have no more knowledge than many of their learners; indeed, they may have less, and/or less experience of life in general. They may very well be less intelligent than their learners.
In Britain, though less so in the United States, I believe, the use of 'sir/madam' often has overtones of address to some form of superior. I, for one, have no wish to be thought of as a superior when I am working with adults on improving their language. If I have demonstrated my knowlege of my subject, and ability in teaching it, then I hope they will respect that, just as I respect their knowledge and experience in their fields, but this does not require 'sir/madam' from either of us
There is also the point that in many cultures and languges, a word such as 'monsieur' (French) or 'pane' (Czech) is used as a polite form of address without any implications of coming from an inferior position. This is not always the case in British English.
I understand why people from other cultures find the British way strange at first, but I do sometimes find it difficult to understand why some people find this particular aspect so hard to accept once it has been explained. When I am in another country, I try to use the courtesy patterns of the country; if I can speak the language, I use the 'monsieur/pane/senor' form of address when it is appropriate. Similarly, I hope that when people address me in my language, they will use the appropiate form of address. In this English language forum, that form of address is nothing at all or our username. I am not upset if somebody addresses me as 'sir' because they initially think that that is polite. I am upset if they continue to do so after I have asked them not to. That is not courtesy.