Service/ experience certificate

Who is the proper authority in a private unaided school to sign an experience letter or an employment letter or a service certificate to an employee while in service? Is it the correspondent (who is the employer) who appoints the staff and fixes the salary ? Does the Principal who is the academic head of the school has also got the power to issue the same ? Can the employer object to it as having been issued without his knowledge ? On whom will the liability fall if the beneficiary employee has misused it while in service ? Who should issue the service/ experience certificate to an employee on termination of his service ?