In the United States, the word "school" describes any place where people learn.
You can call a college a "school." You can even call a university a "school." You can use the word "school" for any English language institute, graduate program, or secondary ("high") school. College or university follows after high school, or secondary school.
A college in the U.S.A. is not a high school or secondary school. College and university programs begin in the thirteenth year of school, when a student is 17 or 18 years old or older. A two-year college offers an Associate's Degree. A four-year college or university offers a Bachelor's Degree. Programs that offer these degrees are called "undergraduate" schools. A "university" is a group of schools for studies after secondary school. At least one of these schools is a college where students receive a Bachelor's Degree.
The other schools in a university are "graduate" (also known as "postgraduate") schools where students receive advanced degrees. Therefore, a university offers both the Bachelor's Degree and graduate degrees such as the Master's (M.A.) and Doctorate (Ph.D.). You can earn a Bachelor's Degree at either a college or a university. However, students in the U.S.A. prefer to use the word "college" rather than the word "university" when they talk about the four-year undergraduate program and the Bachelor's Degree. They say "going to college" and "a college degree" when they talk about undergraduate programs at either a college or a university.
Most "colleges" are separate schools. They are not located in a university. Some colleges are part of a university and are located on the university campus. (The "campus" is the school buildings and surrounding area.) A few colleges offer graduate programs in selected subjects. Usually, however, it is universities that offer graduate programs. So, Americans use the word "university" and not "college" when they talk about graduate study.