I'll second Don's recommendation of putting the mic near the exhaust. You may have to play around with different types of windscreens. The trunk also works well, at least in a Miata. Total protection from the wind, and nice pickup of the exhuast note. Again, play around with windsocks/baffles.
Here's another idea: Use a stereo mic. There are many 12-volt powered lapel/cardiod type stereo mics that are on long enough leads to make this possible. Alternatively, you could place one lead outside the car, and the other inside, incase you are worried about protection from windnoise.
The ultimate, expecially for the instructors, would be to have a stereo mic. Put one lead in the trunk or by the exhaust, and the other in your ear, to pick up the conversation between instructor/student. I did this several years ago and it turned out pretty cool.
Here's a decent 12-volt mic (they also sell bullet cams):
hoyttech.com. I heard about them from MiataSteve, and ended up buying a couple of bullet cams and mics.