a student nurse, medical or dental intern, resident-in-training, student dietitian, student physical therapist, and student occupational therapist, assigned or attached to a hospital, clinic, or medical or dental laboratory operated by an agency; and

any other student-employee, assigned or attached primarily for training purposes to a hospital, clinic, or medical or dental laboratory operated by an agency, who is designated by the head of the agency with the approval of the Office of Personnel Management.