The first writer should not be late for teaching. Turn off your mobile phone, or at least turn it on vibration. You can speak more after the specified time, but it should not be too long, and it is advisable to take ten minutes to a quarter of an hour. Don't be too formal, be generous and decent. Explain clearly that in your university and his junior high school, there are some things you take for granted that he may not understand. Don't look surprised. Tell them again and again until he understands. Be polite.