I know if you don't have a teachers certificate/license most schools will require the coaching certification/classes. I have also heard some schools will allow you to coach with a substitute teaching certificate and count that as a teaching certificate and not require the coaching certification. The only requirement for a sub cert was a 4 year degree (e.g. you do not have to have teaching classes to qualify).
I know if you need to take the coaching certification at the last minute, they do teach a bunch of classes what seemed like every week...so it seems like if you get a job at the last minute that requires a coaching certification you can find a class pretty easily.