Most of the time, it's not massive events that disrupt the good order of a classroom. There are lots of almost inconspicuous steps that you can take just to stop little things getting in the way of your lesson.
Take, for example, the vexed question of the child who has come without a pen (again). Of course you want all of your pupils to come properly equipped and of course you want them to learn how to organise themselves. But what you don't want is a one-minute interlude comprising an interrogation about why the pen has been forgotten - and why it's the fourth time that week. Just keep a store of cheap pens with you, learn to pinch them from wherever you can at every opportunity, and get the lesson going. Any reprimand or prolonged conversation about this can come later.
Another thing you can do when packing your bag is to stick a packet of tissues in there too. Sniffing, spluttering children do absolutely nothing to create an atmosphere of calm, good order, so by quietly slipping a tissue on to their desk you can cut out some unwelcome extraneous noise, pre-empting potentially disruptive complaints such as: "Sir, he's sneezing all over me" as you do so. Preventing such apparently inconsequential matters from becoming time-consuming sideshows is one of the keys to maintaining good order.
And to prevent the lesson becoming a running battle over tucked-in shirts or over-sized earrings, it's a good idea whenever you can to stand in a welcoming manner by the door at the start of the lesson, quietly dealing with these minor infringements in as low-key a way as you can. This also gives you the chance to assess the mood of a class as they arrive, enabling you to adjust your own approach to the lesson if it's possible to do so.
Jon Berry is a senior lecturer in the school of education at Hertfordshire University.