In India, we call "teacher".

In the US, as is the case with many things, it has more to do with your tone of voice and attitude when you say it. Some might consider it a bit impersonal and prefer Mr or Ms Whatever. Yet, your addressing them as teacher in a polite tone of respect, while odd to an American ear would not necessarily be rude. Those with a PhD often prefer the honorific Doctor to teacher or professor in the academic world. As for me and many others, when dealing with adult students I prefer the informality of just using my first name. I hope that was helpful!

In general, it is seen as fairly rude to address someone according to the job that they do. These days, it would not be polite to speak to a waiter in a restaurant by calling him 'Waiter' or to address a taxi-driver as 'Driver'. The only exception is 'Doctor'. You can address a medical doctor as 'Doctor' , because this is also a title. Pupils are often taught to call their teachers 'Sir' or 'Miss', or by their title and surname e.g. Mr Brown, Mrs Brown. In more informal schools these days, teachers are often addressed by their first names.

In my opinion, it would be rude, because it would come across as though you hadn't taken the time to learn your teacher's name. Like kokishiwa said, you should call a teacher Mr./Mrs./Ms./Miss/Dr./Professor [their last name]. In some cases children will use Mr. or Miss and the teacher's first name - for example, in middle school I once volunteered as a craft teacher at a kids' day camp, and the kids called me Miss Amy because I was their teacher but not a grownup. And in some cases, when the teacher requests it, adults or teenagers will use the teacher's first name without a title.

Here we call teachers by their first name. The same goes for university professors. I work part-time in after-school care, and kids who don't know my name call me "teacher" (even though I'm not really a teacher), but that's always sounded really bizarre to me. I don't know if it's specific to the school, the city or the new generation, but when I was a kid "teacher" was never a form of address and I'd never heard it used as such before getting that job. You could say "sir" or "ma'am", but that sounds very formal. It would be more polite to just use their first name, but you should probably ask first to make sure.