Any former member of the state teachers retirement system who was employed as a regular full-time employee in a nonteaching capacity by a board of education, school principal or school administrator, prior to the time he became eligible for membership in the state teachers retirement system, shall be eligible for prior service credit for such service. Upon making application to the retirement board and providing satisfactory evidence, prior service credit shall be granted and his retirement allowance shall be recomputed and adjusted to include such prior service credit. Any increased retirement allowance resulting from the provisions of this section shall not be retroactive.