"Junior" is a term used in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 to describe a student

Student

A student is a learner, or someone who attends an educational institution. In some nations, the English term is reserved for those who attend university, while a schoolchild under the age of eighteen is called a pupil in English...

 in their 3rd year of study (generally referring to high school or college/university study). A Junior is considered an upperclassman. http://www.merriam-webster.com/dictionary/upperclassman

High school

In the United States, the 11th grade is usually the third year of a student's high school period and is referred to as junior year. More electives are offered - for example: AutoCad, Computer Science, Psychology, Sociology, and Shakespeare.

College

In the U.S., colleges generally require students to declare an academic major

Academic major

In the United States and Canada, an academic major or major concentration is the academic discipline to which an undergraduate student formally commits....

 by the beginning of their junior year. College juniors are advised to begin the internship process and preparing for additional education (medical school, law school, etc.) by completing applications and taking additional examinations.

UK Primary School

In the UK any child in key stage 2 (Year 3 - Year 6) is known as a junior. At the end of Year 6 they leave Primary school and go to Secondary School - the transition from Junior to Senior.