Responses
There are men who shave in the morning and by evening have stubble. That's why it's called a "5 o'clock shadow". By 5 pm their stubble is already visible. So, conversely, if sensei shaves in the evening before he meets with the student, by morning his stubble will be visible for classes.
Wait... They met EVERY night...so during day time he's not allowed to show himself? I mean, after all, his day time self has a stubble, and it would be quite different if he'd go to school without it, though he still has those glasses...
I came to imagine a stubble growing overnight.