To teach any subject, I think, sometimes requires one to take a tough edge. However much you may encourage a student, you must also be an honest critic. The balance required is different for different topics; an art teacher can be more of a softie than someone teaching medical school, both because art is a more subjective field and because the results of an artist making a mistake are generally less harsh than those of a medical error.
While there are subjective aspects to martial arts training, I think that at least at the kyu levels grading students centers around fairly objective questions. Does the student know the kata and techniques that they are supposed to? Can they demonstrate the appropriate physical strength and stamina? And does it look like it would actually hurt if they were to hit me?
As for the consequences of letting a student slide, if we take the self-defense aspects of martial arts training seriously (which not everyone does, but I try to), giving someone false confidence can lead to very nasty consequences. And part of what we're supposed to be teaching is a sort of toughness -- not a hardness, necessarily, but the ability to deal with hard situations, which students will not develop if they never encounter those challenges. Furthermore, in traditional martial arts senior students act as exemplars; promoting someone who hasn't met the standards will give junior students a bad model of what their technique should look like.
(That's not to say that standards can't be flexible, set relative to what a person is capable of rather than some absolute. I'm very proud to train in a style of karate where blind students or those with significant physical disabilities can make black belt.)
My first two karate sensei were Marine Corps veterans who had no problem with letting students know, in hard-edged no-nonsense terms, if they were not measuring up. I suppose that from them I picked up a willingness to take the hard edge, to play the Mean Sensei. Sometimes you have to growl, and I think I can do that pretty well.
But you also have to see when growling isn't going to motivate. I hit on an example of that this weekend while I was assisting with promotion testing at my sensei's dojo, working with a girl who was not well-prepared. At first I took it that she wasn't giving it her best effort. So I put on the growly, Mean Sensei face: "Harder! Faster! Louder kiai!" And she rallied a bit, showed better technique, stronger spirit. But it soon became clear that she just did not know some of the stuff that was expected of her.
Part of that is because she was coming back after taking time off due to an injury, which adds an additional challenge -- and we give students credit for that. But part of it was that she just hadn't taken the time to prepare adequately.
If I was dealing with an adult, I might have said, "Ok, it's clear that you're not ready for this. We can stop and you can try again at the next promotion in a few months. We'll work with you to make sure you're ready." With kids, though (and sometimes with adults), we try to take a more gentle approach, while not going soft on standards. If it's at all possible we will allow them to make up the deficient material in the following weeks, either granting them the promotion on a probationary basis or else considering the test incomplete until they can demonstrate the missing requirements.
But before that could happen, she would have to get through today's test without giving up. After she wasn't able to remember a kata she should have known, I gathered her and the other young students I was testing into a huddle, and put on the Nice Sensei face.
"Ok, I'm going to let you in on a secret," I told them. "Because we're all taking this test together, right? We all support each other, that's part of our training. Even though I have to be a little mean sometimes in order to make sure you're strong, I'm also cheering for you.
"We can all see that Jane [not her real name] is having trouble. So I'm going to tell you what you can do when you have a hard time in your training: go back to the basics. Fundamentals. Kihon. For right now, Jane, I want you to forget about that kata and just show me a strong Taikyoku One."
Taikyoku One -- in Japanese, Taikyoku Shodan, but we usually refer to these kata by number -- is the most basic kata, using one stance, one block, and one punch. I had her do it as I counted the steps, and after the first few movements I could see her becoming calmer, leaving behind her mistakes and putting her heart into each punch.
She continued the rest of her test without giving up. She's not finished her promotion; she still owes me a good demonstration of the kata she missed, and I will make her show it to me while I'm wearing the Mean Sensei face, because for it to be meaningful she has to do it under hard circumstances. But inside I'll be cheering her on.