If they use foul language in your lesson, or in your hearing, get them to write down the words they used, and then call their bluff. Send the page to the school office and ask one of the secretaries to photocopy it three times. Then put all three copies on the desk in front of them and say, ‘If I hear you use language like that again, I will put one copy on the head teacher’s desk, one in your school file and send one to your family. Will I need to do that?’ Usually the answer is a very solemn, ‘No.’