I can't help you out suggesting a school but with that said it probably isn't really a must do. The reason I say that is supermotos tend to be very forgiving in how you ride them. First, if you've ridden a bike much you'll be comfortable riding a sumo within 5-10 minutes. I find them to have a very natural riding position, great visibility due to the height and seating position and the controls are all in a comfortable spot for the most part so nothing odd or distracting. Depending on the bike the seat will probably feel like a 2x4 but that comes with the territory.
Now being forgiving in terms of riding, dirt bike guys will ride it like a dirt bike leaning the bike under them, leg out etc where as someone with a sport bike background will tend to lean off the bike and ride how they're used. Best bet is just find what works for you as both seem to be effective. You'll probably end up kind of mixing the styles of riding together using more of a dirt technique at slower speed tight corners and a road race style with high speed sweeper type turns.
Anyways have fun awesome bikes to rip around on. Check out supermotojunkie.com for help with set up and other sumo specific stuff including riding technique.