Any person in the unclassified service employed full time by the Board of Trustees of The University of Connecticut, the State Board of Education, the Technical Education and Career System, the Department of Rehabilitative Services, the Connecticut Agricultural Experiment Station, the American School for the Deaf, the Connecticut Institute for the Blind, the Newington Children’s Hospital, the Board of Trustees of the Connecticut State University System or the Board of Trustees of the Community-Technical Colleges, as a teacher or administrator in a position directly involved in educational activities in any state-operated institution or the Board of Regents for Higher Education, who served prior to such person’s employment by the state in a full-time teaching, administrative or research position in an educational institution in or under the authority of a state department of education or a department of education for the blind in the United States approved by the Retirement Commission, or who was employed by such institution but served all or part of such service time in a foreign country, for which service such person has received or will receive no retirement benefit or pension, may gain credit for such prior service, not to exceed ten years in the aggregate, by making retirement contributions for each year of such prior service equal to six per cent of such person’s annual rate of compensation when such person first became a full-time employee of this state; provided such payment shall be made within one year of such person’s first full-time employment with the state, or before July 1, 1968, whichever is later, but for the Board of Higher Education and Technical Colleges, July 1, 1974. When a person who has gained credit for such prior service retires, not more than one year of such service may be counted for each two years of state service; provided, if such person has purchased more of such service than can be counted, refund on the amount paid on the extra years of service shall be made.