what do teachers actually do when students come in late,

Some get mad and punish the students. Some have compassion and just tell the students to enter early next meeting. If you get angry and punish the students, I think the student will either resent and get mad at you; or they accept as part of their fault. I guess, students may have some reasons if they are late once in a while. But it will be different when it becomes a habit. In any case, find a way to deal with it keeping your composure and good mood!