i am so stupid, can anyone tell me there are 'Brass' teacher that teach all the brass instruments and there isn't many trumpet teacher, trombone teacher, horn teacher......
do you need to play all the brass instruments to be "brass" teacher?

The brass teacher at my school certainly plays all the brass instruments, which is probably the reason the school employs him, as it's very convenient for them. Also, as he runs the brass band, he can help any of the players no matter what instrument they play. However, I don't think it's really a requirement that you play all of the brass instruments if you only want to teach one of them. I know lots of other teachers who only teach one brass instrument.

It is quite common for a brass teacher to teach all brass instruments, but many specialise in lower brass or upper brass. The french horn is very different to brass instruments though so brass teachers shouldn't really teach horn if they don't play it themselves.

At music college, if you're principal instrument is a brass instrument, you should in theory be given some basic training in all brass instruments so that you can teach all of them if you become a peri. My friend has just finished her 2nd year studying horn at the Birmingham Conservatoire, and she's been doing some beginner brass teaching in Birmingham.