Veja o que achei neste site mesmo:
The use of the word "teacher" in classroom
An interesting discussion about the word "teacher" came up in the EMB forum this week (http://www.sk.com.br/forum/display_message.php?message_id=19409). The point of disagreement was whether or not it should be used in a classroom to call the teacher.
In my humble opinion (as I'm not a native speaker) it would sound strange in English-speaking countries. Two other Americans confirmed my opinion, but a non-native teacher of English seemed very confident on the opinion that students called their teachers "teacher" in classrooms all across America.
Please let us know your opinion.
Rodolfo <rodolfosilva6*hotmail.com> Mar 12, 2005
Your opinion is correct and Leah's answer sounds perfect to me. As I have nothing to add, let me just reproduce it here:
The use of the word "teacher" in a classroom setting in English-speaking countries may occur only in elementary school but very occasionally, perhaps only in the first few days of the school year, when the little ones haven't memorized the new teacher's name yet. Never in high school or elsewhere.
Leah on March 10, 2005
This is a typical case of idiomatic contrast and a common interference of Portuguese and for this reason it can easily occur in English schools all over Brazil.
Ricardo - EMB