I was with the Kithener brigade for a while. To be given an instructor position, you will need to have instructor certification. I can't recall if it goes Standard->Advanced->Instructor or Standard->Instructor. And Instructor, other than being a full five days, is bloody expensive. It was $600 last I looked at taking the course. Here's the St John's page (http://www.sja.ca/english/health_safety_training/instructor_dev/), though it contains precious little detail.

But then, the brigade does offer training of its own, or did back in '01.

(Of course, if I'm confused and you're taking the far-more-advanced first aid course, then I'll look really really stupid :)