Do i need a reference letter to apply for a 'z' visa?

Hi, i am a qualified teacher and am in the process of applying for a 'z', unfortunately during the process my contact at the school has changed a few times and each person has asked for different documents. Can you tell me if i require a reference from a previous school to apply for a 'z', please? I hold a bachelors degree, a TESOL certificate and have a clean criminal background check, all of my documents have been authenticated. Thanks for any help you can provide,
Regards
Barry