We had to call our teachers by their title and surname. e.g. "Good morning Mr. Ravenscroft", "Good morning Mrs. Reece", "Good morning Dr. Chin". Most teachers would speak to pupils by using their first names.
We might say to one teacher "Good morning sir", but if there were more than one we would probably use the forms mentioned above. I don't have a problem with using "sirs", if it works in that culture or particular school.
However, well do I remember the miraculous event on my first day in sixth form, where one teacher in particular went from calling us by surname to using our forenames instead.