I like the drivers ed analogy! Then again, how many people sue cigarette companies because they have cancer even though a warning is posted right on the pack? How many people sue a store because they slipped and fell, even though the store has a 'Caution! Wet Floor.' sign?
In todays world, I wouldn't put it past someone to try and hold their inst. responsible. Instructors should always stress to students to use what they've learned as a last resort. Chances are, when they're in court, they're going to be asked if they exhausted every possible option before resorting to physical violence.
I think it would be interesting to hear if there are any such cases out there and if so, what the results were. Like I said, anyone can sue anyone. Whether or not they have a strong enough case is a different story.
Mike