Late arrivals - rarely, occasional, often
I posted earlier that being on time for class involves arriving early enough to get changed and to be on the mats prior to the actual start of class so as to calm down from the frantic pace of the day.
However, the issue of arriving late for any reason really is dependent on the frequency of the occurence.
I have some students who are always early. On occasion they arrive later than their usual time and I find myself concerned that something may have happened to them. The reality may be they arrive at the same time as other students.
I have other students, who though they are seldom late, do cut it close. Many times they are still finishing their knot on their hakama as I clap class to start.
I have other students who will arrive within 2 minutes either side of the start time and then they rush to get caught up.
The fact that I could on any given night provide names for each and every one of them indicates to me that the frequency of "tardiness" is the main factor. Since I have managed to perfect the "parental evil eye" (those of you with children know the one, and those of you who are someone's child have probably received its' gaze at least once), I do not hesitate to use it when a person is consistently late.
That is the real issue. If one is late once a year due to unforeseen circumstances, I do not know many Senseis who would chastise them. If one does so once a month, then perhaps you need to examine why and maybe choose to do it differently.
If one is chronically late in arriving, then perhaps one needs to become better organized or look for a different schedule. Perhaps your sensei needs to have a different schedule.
However, unforeseen circumstances aside, it is the student's responsibility to make every effort to be seated in seiza prior to the start of class.