In 451 or 450 Pericles carried a law confining Athenian citizenship to those of Athenian parentage on both sides. No source provides any background to this proposal; it is not even clear whether it was retrospective.
A high school diploma or GED is enough to get started as an office clerk. It is important to have some office skills, such as typing, good phone skills, word processing, etc. Most training is done on the job.
Some employers may expect you have GCSEs (A* to C) including English, maths and science. Experience of office work may help you get a job. You could get into this work with: an HND, degree or postgraduate qualification in human resource management.