God save the Queen

I was wondering why this expression is not “God saves the Queen”. According to my very first English teacher, when the subject is he, she or it, “to save” is conjugated “he/she/it saves”.

Is it an exception?

It is not an exception. This is the subjunctive mood, being used to express a wish. See “Third person requests with a main-clause subjunctive” in the Wikipedia article on “English subjunctive”, which gives “God save our gracious Queen” as a specific example.

You may think of it as short for “May God save the Queen” or “Let God save the Queen” if that helps you parse it better.

It’s because save is subjunctive. In particular, it’s an example of the ‘formulaic subjunctive’, found in other fixed expressions such as Heaven forbid and come what may. It is used in God Save The Queen to express a non-factual concept: we cannot assume that God saves The Queen, but we express the hope that he will.