For example, consider that we have the National Association of Head Teachers that represents heads but the standards they operate under are the "National Standards for Headteachers" published by the National College for Teaching and Leadership. The leading institutions cannot even agree which is correct.
|The association the represents heads says it is two words.|
|The standards they operate under uses only one word.|
Therefore the answer is: either is considered to be acceptable and correct. Best practice is to look up the head on the school website and copy their spelling in a letter or check how it is used in the context that you are using it. Otherwise you can just pick one (personally I prefer headteacher as one word). Additional considerations to bear in mind is that:
- Not all heads are headteachers/head teachers. They may be principals or, in Scotland, Rectors.
- Professional organisations, documents or specific heads should be referred to with the same spelling as they refer to themselves.
- In my experience headteacher is more common than head teacher.
- The head of single-sex school is usually the Headmaster or Headmistress rather than headteacher.