I think this varies among different places. I just provide the situation of where I train.
I train Taekwondo in Hong Kong. My master is a Korean, so I guess this is quite a traditional way. In my school there is only one master 師父 (師=teacher, 父=father) and all of us are his students. All of us call him master.
For those who teach, we call them 師兄/師姐 (兄=elder brother, 姐=elder sister). The term 師兄/師姐 has the meaning that we are all siblings, fellows of the same generation.
If the one who teaches is an adult and those who learn are young kids, they call him 教練 (instructor/coach).
