By the time a child reaches their final years in Primary school, they will be totally familiar with their school environment. They will know their teachers and classmates very well and will probably know most of the other faces in the school too. By Year 6 (in a British school following the English National Curriculum), they are the most senior people in their school and tend to be looked up to by the younger students.
Within a few months, they revert to being the most junior again in a brand new social environment when they transition to Year 7 in Secondary school. Change can be a daunting task for any one of us but when you are 11 or 12, it is a change that takes quite a lot of adjusting to.
The important thing to remind yourself is that children are more resilient than we are. Remember too, that typically in a short while, their new surroundings will become more familiar and they will be running from the gym to the Science Lab and on to the Art room without the slightest stumble. The first couple of weeks can be difficult though.