These are all words for a child who attends school.
studenta person who is studying in a school, especially an older child:
Students are required to be in school by 8.30.学生须在 8:30 以前到校。
Any high school student could tell you the answer.随便一个高中生都可以告诉你答案。
pupil(British English) a person who is being taught, especially a child in a school:指学生，尤指小学生：
The school has over 850 pupils.这所小学有 850 多名学生。
Pupil is used only in British Englishand is starting to become old-fashioned. Student is often preferred, especially by teachers and other people involved in education, and especially when talking about older children.
schoolboy/schoolgirl/schoolchilda boy, girl or child who attends school:
Since she was a schoolgirl she had dreamed of going on the stage.自从上学以来她就一直梦想着成为演员。
These words emphasize the age of the children or this period in their lives; they are less often used to talk about teaching and learning: an able schoolboy/schoolgirl/schoolchild