USUALLY, what happens is that the teachers mark it and then they send the best one and the worst one and a few others off to be moderated. The exam board checks the marking and decides whether they've marked them well, whether they've been to lenient / too harsh, or whether they need to see some more to decide. If the exam board decides that your school has been too lenient in the marking, then they will reduce everyone's grades by a certain amount. If the exam board decides your school was too harsh, then they'll increase everyone's grades by a certain amount (capped at full marks of course)