D.C. Code § 38-2001.08
In computing length of service of retiring teachers credit may be given, year for year, but not to exceed 10 years, for public school service or its equivalent outside the District of Columbia; provided, that no credit for service outside of the public schools of the District of Columbia shall be given to any teacher entering the said public schools after June 30, 1926, until he shall have deposited to the credit of the teachers' retirement fund of the District of Columbia a sum equal to the contributions that would have been required of the teacher if such service had been rendered in the public schools of the District of Columbia, with interest thereon at 4% per annum, compounded annually, said contributions to be based on the average annual salary of the class to which the teacher is appointed; provided further, that when the average annual salary of the class exceeds $2,000 the contributions shall be based on a salary of $2,000; provided further, that if the teacher so elects he may deposit the required sum in the fund in any number of monthly installments not exceeding 100, with interest at 4% per annum, compounded annually; and provided further, that the provisions of this subchapter shall not apply to any teacher who receives an annuity from any state or municipality other than the District of Columbia, nor to allow any teacher more than one year's credit for all services rendered in any one fiscal year.
D.C. Code § 38-2001.08