OK, staffroom-wisdom-unconfirmed-rumour-come-government-dictat: OFSTED inspectors will mark you down in a lesson observation if you aren't nice to late comers. At the post-16 level, when they don't have to be in school all the time, lateness to lessons is a huge issue. But according to official rumours, you have to make sure that latecomers are helped to settle in to the lesson, that you sit them with a nice kid to help them get on stream as quickly as possible, and the teacher should make sure that they end up getting all the content they missed by being late, presumably by taking time out from teaching the others or maybe staying late at lunchtime to help them catch up.
So, kids, as part of your preparation for adult life, we will incentivize lateness by rewarding it with private tuition and extra one-to-one input, while students who make the mistake of arriving on time and ready to learn will be disincentivized by being made to sit next to their most irritating and feckless classmates. Ah, OFSTED.