I just wanted to add one non-acrimonious addenda to the comments. This is coming from one of those crazy guys who rode fixes on the streets of Manhattan. I found, fwiw, that the main trick was staying in control. I wouldn't allow myself to descend a hill faster than I could slow down with my legs. Stopping, per se, was not the idea, except in an absolute emergency when, ime, I could stop almost as well as someone using their brakes. So, in one sense, I think it'd be helpful to new riders to learn correct slowing and stopping technique --oh, and appropriate gearing. Using a gear that one can't control on a descent can be dangerous.
I have to say that I'd probably have a new rider be wary of using the back brake. That is, unless his/her legs are "coasting." (Yes, I realize that coasting isn't really possible on a fixed wheel, which that gets me to the next point). If back and front brakes are used, isn't a one-speed coaster set up a preferable idea? That's especially true for descending where, imo, letting the legs go and depending on the brake could cause loss of control. (Like the first time fix rider who forgets and tries to coast. If it's happened to you, I think you probably know what I mean.)
Again, this isn't a criticism of anyone. I'm skeptical of the riding towards a wall "test." I wouldn't ride my road bike full speed toward a wall and expect to stop in 10 feet either. It would be interesting to see some real comparative data on stopping distance. Does any exist?
regards,
Steve James
Bronx, NY USA