Why do Americans say they're going to school when going to college or university?

A school (from Greek σχολή (scholē), originally meaning "leisure", and also "that in which leisure is employed", "school"), is an institution designed to allow and encourage students (or "pupils") to learn, under the supervision of teachers.

In addition to core schools, students in a given country may also have access to and attend schools both before and after primary and secondary education. Kindergarten or pre-school provide some schooling to very young children (typically ages 3-5). University, vocational school, college or seminary may be available after (or in lieu of) secondary school. A school may also be dedicated to one particular field, such as a school of economics or a school of dance. Alternative schools may provide nontraditional curriculum and methods.

https://en.wikipedia.org/wiki/School

Thanks for fixing my little error!

Specifically I'm referring to the practise of calling where they go to do a degree school as well. I've never heard this outside Americans and wonder why they don't use college instead as it's very confusing for people from elsewhere.

You're most welcome!

As I stated, a school is an institution designed to allow and encourage students (or "pupils") to learn, under the supervision of teachers.

A school is a place for education; a place where people go to learn -- In the U.S., we casually use the term to cover all institutions of learning, regardless of level or type...a generic term, if you will. Similarly, we will say "I'm going to church," which can refer to any of the churches...or..."I'm going to the store"...etc.

http://www.dictionary.com/browse/school

It's even more confusing if they call the institution where they study for a degree a College - when I moved to Australia, my degree from Smith College was initially understood to be a secondary certificate, as a college in this country used to refer to a fairly elite secondary school. Now the phrase Secondary College is ALSO used to describe the secondary schools formed when academically-oriented and vocationally-oriented secondary schools amalgamated during the egalitarian reforms of the 80s and 90s. To get a Bachelor's Degree, you must attend a University. (And, if you live in an on-campus residence, it will probably be referred to as a college!)

In the U.S. the terms are synonymous; other countries use "college" to refer to some secondary schools, but "university" is always used to mean an institution of tertiary education and higher learning.

http://wiki.answers.com/Q/What_is_the_difference_between_college_and_university

In the U.S., in order to differentiate, one may need to look at the actual name of the institution:

University of Northern Colorado was changed from Colorado State College in 1970 -- Both were four year programs.

https://en.wikipedia.org/wiki/University_of_Northern_Colorado

Colleges, Universities, and Institutes: The Distinction -- Degree-granting institutions in the United States can be called by any of these terms, and colleges and institutes are in no way inferior to universities. As a general rule, colleges tend to be smaller and usually offer only undergraduate degrees, while a university also offers graduate degrees. The words "school," "college," and "university" can be used interchangeably. An institute usually specializes in degree programs in a group of closely related subject areas, so you will also come across degree programs offered at institutes of technology, institutes of fashion, institutes of art and design, and so on.

Within each college or university you will find schools, such as the school of arts and sciences or the school of business. Each school is responsible for the degree programs offered by the college or university in that area of study.

In the UK it's school until 18, college for non-school institutions, including for degrees, plus some private schools call themselves colleges. But it would be very unlikely for anyone to say they're at college as that refers to places either taking pre-degree exams outside school (eg for adults) or a degree and beyond.

School here is never used outside the specific place we have to go to by law until 16, but now understand why it's been stretched to include all the places we go to after we've finished school. It did seem strange to see people clearly in their 20s on TV in America to say they were at school to then find out it was college.