+1 This has been suggested a few times, but I agree that maybe a forced quiz that newbie cops must take, like rules and what to do in "X situation".
Failing any, and I repeat ANY of these questions will demote/remove them from cops until they get it right. You may say that failing only one question is a bit extreme, but if a cop gets something wrong, it shouldnt let that error affect his duty in anyway.
I think that this quiz should be aplied on the begining, and then a few hours of cop duty, just to check if they remenber the rules.