Okla. Stat. tit. 70 § 18-111
No pupil shall be counted in the average daily membership of any district for the purpose set out in this article unless said pupil is a legal resident of said district as provided for in Section 1-113 of this title or has been transferred to said district. A pupil moving from a school district during a school term shall be entitled to attend such school for the remainder of that school term. School districts shall not include out-of-state pupils in their average daily membership for the purpose set out in this article. The following pupils shall not be counted: Those who have attained twenty-one (21) years of age by September 1 of that school year, or who have completed the twelfth grade, except as elsewhere provided for by law for veterans of World War II, the Korean War, or the Vietnam Conflict, and those who have not attained four (4) years of age by September 1 of that school year. In determining the ages of pupils for State Aid purposes, birth certificates shall be presented, if obtainable, as proof of age. Underage pupils in kindergarten and first grade who have been in legal school attendance in a public or private school in another state or in a Department of Defense School for military dependents may be legally enrolled and attend an Oklahoma school.
Okla. Stat. tit. 70, § 18-111