I've done Skip Barber (formula car), the Porsche Driving Experience (911/996) & SCCA (Spec Racer Ford) schools.
Make sure there is a seat in the car for the instructor. Getting feedback every 20 minutes is not that helpful. Having, for example, a Le Mans winner sitting next to you telling/showing you how to move the car around is a very humbling experience.
I agree with Kevin (what else is new
), high HP isn't necessary to learn driving skills. In fact, I would go so far as to recommend a low HP car if it's your first time out.