What is the abbreviation for headteacher?

There isn't a widely recognized abbreviation for "headteacher." However, some people might use "HT" informally.

It's important to note that "HT" can also stand for other things, so it's best to avoid using it unless you're sure the context is clear.

The best practice is to use the full term "headteacher" or, depending on the context, "principal" or "headmaster/headmistress."