Connotation
I would like to explain the different connotation of "teacher". In Japan when we call a teacher, we usually say "sensei". It is very polite and respectable the way to call the teachers and it's normal, however, some naughty students call the teacher "senkou". It is very unrespectable way to call them and the students make fun of the teachers, but sometimes even normal students call them "senko" behind the teachers, for example they say it when they talk about a teacher who they dislike or when they speak ill of a teacher.