It's until the child reaches the age of majority, dies, marries, or becomes emancipated, whichever happens first. If the child reaches the age of majority (18) and is still in high school, support continues until the child graduates, yes, but the cutoff age is 20. Once they reach 20, even if still in high school, support stops. In a nutshell, if they turn 18 before graduation, it continues until graduation. If they turn 18 after graduation, it continues until age 18. If the child marries at any time before age 18, the CS stops.
"O.C.G.A. 19-6-15
(e) The duty to provide support for a minor child shall continue until the child reaches the age of majority, dies, marries, or becomes emancipated, whichever first occurs; provided, however, that, in any temporary or final order for child support with respect to any proceeding for divorce, separate maintenance, legitimacy, or paternity entered on or after July 1, 1992, the trier of fact, in the exercise of sound discretion, may direct either or both parents to provide financial assistance to a child who has not previously married or become emancipated, who is enrolled in and attending a secondary school, and who has attained the age of majority before completing his or her secondary school education, provided that such financial assistance shall not be required after a child attains 20 years of age."
And like LdiJ said, you do have to petition to stop the CS.