If a disability beneficiary under the age of sixty-five years is restored to active service as a school employee or if the examining physician certifies that the person is no longer disabled for service as a school employee, the disability retirement allowance shall cease. If the beneficiary again becomes a school employee, he or she shall become a member of the retirement system. Any prior service certificate, on the basis of which his or her creditable service was computed at the time of his or her retirement for disability, shall be restored to full force and effect upon his or her again becoming a member of such retirement system.