...not that belts or gradings are remotely traditional (katas are traditional though ). And while the grading/belt system might sift out some of the thugs, it won't filter them all out. I've personally come across two people Karateka - one was a brown belt who was nearly on his black belt, and the other had acquired his black belt as a child. Both are very angry and violent people who, IMHO, should never have been given martial arts training (but they have because they've been very patient and persistent in their training).

In my personal observation, I find that the belt/grading system really tests a student's patience rather than their ethics. People who drop out aren't necessarily people who are thugs, but just people who have become dissatisfied with the training, or for whatever other reason have chosen to stop training. Now of course, a lot of people with poor attitudes also happen to be impatient, but as my experience has shown me, there are also angry/violent people who are willing to be patient and wait it out.

A far better way to sort students out is just to rely on instructor's discretion. When you train with people regularly you get to know their personalities. You yourself know who are the respectful and who are the disrespectful students in your Dojo. You don't need any belt/grades to tell you that! Just by talking with them and watching how they train, you can see what kind of person they are -- you get to see their personalities and attitudes. As if you need to put ALL of your students through rigorous gradings to tell who the jerks are.

Quite frankly, schools should just approach students with the inappropriate attitude and either expel them, or modify their training so that they're not learning any new techniques for hurting people. Disrespectful and irresponsible people who are trained in martial arts only serve to tarnish the reputation of all martial artists in the community (let alone bringing disrepute to your school).