【答案解析】本题为细节辨认题。文章第二段第二句“Some independent schools are called prep schools because they prepare the children for the Common Entrance Exam which they take at the age of 13”中，定语从句“which they take at the age of 13”说明：学生在13岁时将参加the Common Entrance Exam(普通入学测试)。