께, 께서, 께서는 are case markers used for someone of higher age or authority. I will show the equivalent neutral markers. * 께 = 에게, 한테 ("to" in English). 선생님께/선생님한테 말씀드려 = Tell the teacher. * 께서 = 이/가 (subject marker). 선생님께서/선생님이 이렇게 말씀하셨어 = Our teacher said this. * 께서는 = 은/는 (topic marker). 선생님꼐서는/선생님은 오늘 못 나오신대 = They say the teacher won't be coming today. The difference between 께서 and 께서는 is the same as the difference between 이/가 and 은/는. Roughly, the former is used to describe something originating externally (an event that happened independent of the speaker's thought), and the latter for 1) things that the speaker had in mind, or 2) when they want to state it contrasting it with other cases. So 선생님께서 expresses that the event just happened on its own - i.e. an event the speaker wasn't expecting. 선생님께서는 implies that this is not the usual thing, meaning that everyone else, or the teacher himself on other days, comes to school.