how many years of high school are there

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.
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.

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.
To receive a regular high school diploma, all students enrolled in a Tennessee public school during their eleventh (11th) grade year must take either the ACT or SAT. View the FAQ on the policy here.

References:

http://www.tn.gov/education/instruction/graduation-requirements.html
http://www.toppr.com/guides/essays/importance-of-education/