For me, it's the overall effect on the class. New students take time away from the other students, just by virtue of increasing the class size. But they provide new examples, new training partners, and potentially eventually future leaders.
However, if a student takes more than their fair share by being disruptive or disrespectful, or if they're a bad training partner, then it becomes a problem. Some of this can be part of the lesson. But some is so egregious, it's hurting all of your other students to keep them.
I remember a lot of students that were frustrating. I only remember one that I would have kicked out if given the opportunity. There may have been a few more, but it was few and far between.
The one I remember was 12 years old when he started. But a tall 12. He went into the 8-12 class and felt out-of-place because everyone else was so much smaller than him, and wanted to go to the 13+ class. I thought he was out of place because everyone else was so much more mature than him, and he belonged with the 4-7s.
This kid did go into the teenage class. During warmups, when he was supposed to count, he would alternate between whispering and screaming. There was one kid that he would bully and threaten. Which was especially stupid, because the kid he was bullying was twice his size and much, much, much, much better at Taekwondo. There were other students that he liked, so he would distract them during class.
He would go 0 effort on most techniques. If he put any effort in and did something anywhere close to correct, and receive any small amount of praise, he would go, "Yeah I'm the best I should be a black belt." I'm not sure if he was joking or not.
Nobody wanted to partner with him. When the Master would assign a black belt to him, I could see them grimace. Even black belts that were typically respectful of everyone. The other folks in the class? Even worse. He would spend half the time talking, and most of the remainder goofing off. I remember watching one punch defense, where the other person stood there for about 30 seconds with their hand out before he responded, because he was too busy talking.
The one time we did enjoy having him in class was the day he got his sparring gear, and black belts were lining up to kick him as hard as they could. I don't know that anyone blamed them, even the kid's Dad, who was at wits end with him. The Dad was one of the kindest, most respectful people I've ever met. I don't know how that kid came to be.
To my Master's credit, he wasn't just letting this kid get away with it. I would say that 90% of the time, he was called into the office after class, and he would often be in there for a half hour or so, while I led the next one. My Master was really trying to reach him. But, in doing so, it meant that the one kid got bullied, the other students got a bad training partner, black belts hated helping out, and then the Master was unavailable for most of the next class, all because of this one kid.
