I have had two sons sit the exam at SHSB. It was very well organised. When you arrive and go through the front entrance there are lots of friendly, approachable sixth formers and staff on hand to tell you where to go. We were directed into the dining hall first where they both checked in. They were then taken to the toilet in small groups and brought back to the check in point. DS2 sat the exam in the main hall, I believe, and DS3 in one of the classrooms.
They were allowed to take a transparent pencil case into the exam but no drink or snack. They will be provided with that during the break. Both sons said they were also escorted to the toilet again.
There was a boy crying when DS2 sat the exam and I have heard reports of children being sick but have no experience of it myself. I warned mine that this may happen and told them to ignore whatever else went on and to concentrate on the exam. One other tip is to tell your DS not to take any notice of boys who are talking about how easy the exam was, during the break. It is just bravado but I didn't want it to worry my sons. Good luck!