A teacher may indeed get fired for having a mental illness. If the teacher was hired and lied on the application and events subsequent to the hiring showed the teacher deliberately lied, then yes, that is an offense that offers cause for termination. If the teacher did not lie, was hired and had or has episodes that impact his or her ability to teach or place the children or faculty in danger, that is an offense that can give rise to termination and I don't believe it would be considered discriminatory in nature. The teacher, if fired, doesn't mean the above scenarios can absolutely sue for discrimination but would most likely have to get the teacher's union involved for such an event.