Is it possible to get subcategories of a category by tag in wordpress?

I am developing a custom application using wordpress, Here there were student login when they login they can see all the semester which created as tags, course and subjects are category and subcategory. I need to list all the semesters, under each semester their subjects need to list. how is it possible,

semesters were the tags, Course were Category, Subjects were subcategory

eg: if there are 6 semesters for BBA course. List all the 6 semesters from 1 to 6. ie can display tag in wordpress. But how to display subcategory ie the subjects under semester