Within the United States, there are variations from state to state and community to community. There are no fixed rules. I will describe ONE common pattern. All number, ages, are only approximate. 

Public school is grades "K through 12." It includes elementary school, high school, and something in between. Kids enter kindergarten at age 5 and graduate from high school at age 18. They have "a high school diploma." 

About 85% of the U.S. population over age 25 receives a high school diploma.

High school is never called "college." College always follows high school.

A "college-educated" person has had four years of study after high school. 

A college can be a school in itself (Amherst college). A college is often part of a university. Students who graduate from a four-year college usually receive a "bachelor's degree." They are "college graduates."

At a university, the students who have not yet received a bachelor's degree are "undergraduates." The college within the university is "the undergraduate school." The college also has "graduate schools." These award "advanced degrees:" the master's degree and the doctorate or Ph.D.

Hi!I'm gonna try to explain you my viewpoint about topic:There are three larges level in education,the first is elementary school for children aged from 6 to 10-year-old.The second is high-school for underages(junior and senior) until 17-year-old and the third is college or university from 18-year-old until graduation. 

Thank you very much, guys. Now it's really clear!