If you're willing to drive an automatic, BMW M School is great. I did a two day course and a one day course out near Palm Springs. Looks like they do Indianapolis too:
BMW M Driving School | Advanced Driving Performance School
You get to drive M cars as hard as you want all weekend. I broke an M2 (the most fun of the group) and no one was upset with me - just part of the experience. Although maybe a BMW breaking isn't a novel occurrence.
They do spin out recovery training on wet parts of the track, slalom, high speed track, emergency braking - it might be the most fun you can have with your pants on.
Porsche driving school is worthwhile, although Porsche's have an odd amount of understeer so it won't be directly applicable to your M6 but still a great way to improve control and precision. Maybe you'll enjoy it so much you'll keep the Porsche and sell the BMW. That M6 is terrifying. Porsche schools are in Alabama and Atlanta since California isn't an option:
Home | Porsche Driving
If you sign up for one, Ben and I will join you. Don't eat before riding as a passenger with one of the instructors, I learned the hard way.