u can sit an exam at another school but only on valid reasons---e.g u live an hour away from your school and there is a car accident blocking the road. then u can sit the exam at the local school. u have to notify them and your school (obviously) and u cant just sit the exam there coz u cant be bothered to travel the whole distance to school.
unless its due to an "illness/misadventure" you have to have already sought permission from your marking centre, the other marking centre and the Board of Studies...