Is it permissible for a female teacher to take leave from school to perform Umrah, knowing that she has completed all her curricula and is entitled to sick and casual leave?

The reference for this is the school system, labor law, and the agreed-upon conditions at the time of contract. If the responsible administration approves granting a leave, there is no harm in utilizing it. The employee must adhere to the terms of the contract, as Allah Almighty says: "O you who have believed, fulfill [your] contracts," and the Prophet, peace and blessings be upon him, said: "Muslims are bound by their conditions." One must not deceive the administration by falsifying facts, such as taking sick leave without being ill, as this is forbidden deception.