how many years is high school

In the USA, children start school when they are five or six years old. Depending on the state, schooling is compulsory until the age of 16 or 18. Children younger than five can go to a nursery school or preschool.
At the age of five or six, the children attend elementary school (also known as grade school or grammar school), which last six years. The fist year at elementary school is called kindergarten.

Following the implementation of the Tennessee Diploma Project in 2009, high school students must complete 22 credits to graduate. They also will be tested in core subject areas with End of Course exams, part of the Tennessee Comprehensive Assessment Program, or TCAP. Their performance on these exams will factor into their semester grade for the course.
The Tennessee Department of Education has raised standards and aligned graduation requirements to best prepare students for college and the workforce.

References:

http://www.tn.gov/education/instruction/graduation-requirements.html
http://www.goodschoolsguide.co.uk/choosing-a-school/independent-schools/uk-independent-schools-explained