It is often convenient to have a student clear the board for you when you are getting the class ready to leave for recess or lunch. Have a student assigned to this class job and have them erase the board(s) while you line up the class for lunch. They should then join at the end of the line. At the end of the day, when students are getting ready to go home, they should take the erasers outside and clean them. Make sure to rotate this job weekly.