When a down trodden high school chemistry teacher receives a terminal diagnosis he decides to cook crystal meth in order to support his pregnant wife.

It’s good but you’ve mixed up the event and the character. First you must identify the event or the ‘when a…’ statement and then introduce the character or ‘a…’. For this logline the better structure would be “When he receives a diagnosis for terminal cancer, a high school chemistry teacher must cook crystal meth in order to support his pregnant wife”