I was recently exposed to a similair situation... a friend who admitted (boasted) to me that he was using our clubs 172's to (attempt to) perform aero's, sometimes fully loaded (ie 4 POB).
VIPER, this is extremely serious. If you haven't gotten the idea yet, from the other posts in this thread... THIS IS EXTREMELY SERIOUS. Both you and this instructor of yours are heading down a road labelled either "Death" or "Manslaughter".
I suggest, if you can not bring yourself to report this to the relevant authorities, that you do what I did. Write an anonymous letter to the aircraft owners. Don't name names, but make the situation clear. Hopefully they are smart enough to treat these concerns seriously, and to do something about it. A simple education programme (notices posted in briefing rooms, warnings in the club magazine etc.) certainly stopped by friend from doing these stupid things.
VIPER, you must excercise responsibility in this situation. Don't do it, and take action to help insure that no-one else does it either.
Stall turns in a 172... sheesh!