Full Centre Supervision means that students can be in the examination room doing their examination under the watchful eye of invigilators. Or, if they have finished their examination, they may be permitted to leave the examination room provided they are supervised by invigilators continuously from the time they leave until the Key Time. Once outside the exam room, they can have access to their books and notes, and they can talk to each other (provided they do not cause any disturbance to students who are still doing an examination). They must not have any access to telephones, mobile phones, the internet, or any means of external communication; and they must not be allowed any contact with anyone else not under Full Centre Supervision.
For more information, please refer to the 'Key Times and Full Centre Supervision' part of the Cambridge Handbook, which can be found here.
Our interactive tutorial here explains some of the main regulations for timetabling examinations and will assist examinations officers in implementing them in their school.