If I'm not rehearsing/on the podium, I'm a sarcastic goofball that jokes around and laughs with the kids. They never know what to expect from me, and I'm ok with laughing at my self and the goofiness that comes with being a teacher.
Once I step on the podium, I try to channel my inner J.K. Simmons from Whiplash, without the vulgarity. I'm intense and blunt on the podium, but never mean. If a kid doesn't know their notes, I will tell them "You need to learn your notes", but its less of a personal dig and more of a fact. If the group does something that I know was due to a lack of effort(sloppy release, unsupported sound, missed attack) I will stop them and say "That was bad. You're better then that. Do it again" and they typically fix it.
Ultimately it boils down to this... play more, talk less, work hard, play hard, and have fun.