I don't understand how the rhythm of the class is broken by a late student. In our dojo, the late student sits in seiza out of the way of the other students on the mat until given permission to bow in by the instructor.
I agree. I was rather amused by the idea that someone's practice might be disturbed by someone coming in and sitting quietly next to the mat until the instructor beckoned him onto it.
My first child is just over 2.5 years old. For the last year, he and my wife have sometimes come to the dojo when I practice. When he gets too exuberant, she takes him outdoors. After the first time he made noise, I checked with my sensei to be sure that it was all right. He assured me that it was, and said quite flatly that if his students were distracted by such a thing, then they needed to learn how to focus. He encouraged me to bring my son whenever I liked. (The other students were equally encouraging, and they all have fun with him when he runs around on the mat after class.)
I think the bigger issue of being late is that it could be construed by the instructor as the late student showing a lack of respect for the instructor or a lack of enthusiasm for attending class (notwithstanding the reason for the tardiness, valid or not). IMHO
It is certainly the obligation of the student to explain lateness to sensei. Recently, my work schedule has been changed so that I am off duty at the instant that class starts, and class is half an hour away. This is very frustrating for me, because I miss some class and because I feel a responsibility to help clean the mat before class. My efforts to flex the schedule were not successful, and so I approached my sempai and sensei and explained the situation. They were very agreeable and said that I was still getting at least an hour on the mat, and that I should just get to class as soon as I could, safely.
I also explained briefly to the dojo at large after class one day, so that they would not think that it was willful on my part.
Thus, our communication was good: I explained that I had an unavoidable conflict, and they accepted that and gave me support in accomodating as best I could.
Let's suppose, for the sake of argument, that a teacher notices a student arriving half an hour late pretty regularly. If the teacher feels that it is a problem, it seems to me that good results are most probable if he communicates openly. He should approach the student and ask why he is arriving late. If the student's explanation is acceptable, then the discussion remove a source of irritation. If the student's explanation is not acceptable, then the teacher can teach the student why and await improved behavior.
It also seems to me that the difference of opinion between Tomu and Chris Li is illusory. Although Tomu's tone has struck me as rather strident, he has made it plain in later posts that he is discussing a situation where the student has no good reason for being late. I infer from Chris Li's posts that he is discussing a situation where a teacher sends a student home peremptorily, without offering the courtesy of inquiring as to causes before issuing edicts.
I also agree with Crystalwizard when she says that students who are habitually late either reform eventually or stop coming, and thus are not a problem in the long run. Let me add that I think it important that their example not be permitted to stand as acceptable. The disapproval should be apparent to students who care to look, so that they do not themselves offend. As always, that disapproval should be expressed courteously.