Is it farz on every muslim girl to wear niqab/burka when she has reached age of puberty?

Answer

A female is something qualified as "awrah" according to Shariah. Awrah means something that should be concealed. Hence female should wear niqaab at the age of puberty. It is a sign of dignity for female.

Waleemah or any mix gathering is prohibited according to Islam because it is against the law of pardah. These types of gathering lead to indecency and immorality.

On the same basis working for girls is not allowed, unless when the pardah can be observed easily like venues and premises restricted to women only.