My old vet teaches equine dentistry at University of Gainesville (Central Florida). I believe you need to have a vet degree to tranquilize them - but if you could arrange for a vet to tranquilize horse then do other work while you work on horses mouthes then you'd have a match made in heaven (most vets I know do not like working on teeth). My vet will soon be getting an assistant since he's getting older and the physical work is getting to his legs and back.
He wears a headlamp so he can see in the back of the horses mouth. You really do NOT want the schools that hang the horses head from the ceiling - instead look for those which have a Y shaped adjustable height device to rest the horses head on - this is more natural and safer in case they lose their legs from tanquilizer - you want Y device to fall with horse, not have horse hanging from ceiling by it's neck!
Like photo's here - a few of the pics from this Virginia based school show the device ASED Photo Gallery
Last picture show "speculum" which holds the horses mouth open while dentsist are working on it. You'll want to learn from someone who knows they need to provide occassional "relief" from the speculum - you wouldn't want to be totally awake for hours with your mouth pried open - so horse should get a break every 15-20 minutes or so.
These guys also use that Y shaped device: Advanced Equine Dentistry - Services - Equine
Some of these "schools" appear to be VERY pricey - some may have beginner courses for people who don't know the basics of handling horses, etc. So perhaps get recommendations from working equine dentists - how much does graduating from these "schools" help you get a job post graduation?
Think about that before plunking down a bunch of money.