Junior is a term used in the United States to describe a student in their 3rd year of study (generally referring to high school or college/university study). A Junior is considered an upperclassman.
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.
In the U.S., colleges generally require students to declare an academic major 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.