38 USC 3221 - Eligibility

(a) Each person entering military service on or after January 1, 1977, and before July 1, 1985, shall have the right to enroll in the educational benefits program provided by this chapter (hereinafter in this chapter referred to as the program except where the text indicates otherwise) at any time during such persons service on active duty before July 1, 1985. When a person elects to enroll in the program, such person must participate for at least 12 consecutive months before disenrolling or suspending participation.
(b) The requirement for 12 consecutive months of participation required by subsection (a) of this section shall not apply when
(1)  the participant suspends participation or disenrolls from the program because of personal hardship as defined in regulations issued jointly by the Secretary and the Secretary of Defense, or
(2)  the participant is discharged or released from active duty.

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(c) A participant shall be permitted to suspend participation or disenroll from the program at the end of any 12-consecutive-month period of participation. If participation is suspended, the participant shall be eligible to make additional contributions to the program under such terms and conditions as shall be prescribed by regulations issued jointly by the Secretary and the Secretary of Defense.
(d) If a participant disenrolls from the program, such participant forfeits any entitlement to benefits under the program except as provided in subsection (e) of this section. A participant who disenrolls from the program is eligible for a refund of such participants contributions as provided in section 3223 of this title.
(e) A participant who has disenrolled may be permitted to reenroll in the program under such conditions as shall be prescribed jointly by the Secretary and the Secretary of Defense.
(f) An individual who serves in the Selected Reserve may not receive credit for such service under both the program established by this chapter and the program established by chapter 106 of title 10 but shall elect (in such form and manner as the Secretary of Veterans Affairs may prescribe) the program to which such service is to be credited.