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.
Clerks typically get training on the job from more experienced clerks, accountants or financial supervisors. They learn how to use accounting software. They also learn double-entry bookkeeping, which records transactions twice, as a credit and debit, to ensure the balancing of accounts.