Can I introduce dialogue as follows -
The headmaster entered the room: "Everybody sit down now!"
or -
"Everybody sit down now": The headmaster had entered the room
I know I could say -
The headmaster entered the room and said, "Everybody sit down now!"
but I am exploring acceptable alternatives
While technically valid, using a colon here doesn't really work. Segmental colons in prose have largely fallen in to disuse and these days are largely the province of scripts. I don't think it's a huge problem though as arguably you don't need any dialogue tag here:
The headmaster entered the room, "Everybody sit down now!"
This works just as well - the fact that the headmaster was the last character to hold focus, along with the general context provides enough for the reader to understand that it is them speaking.
As a writer, sitting before your keyboard or other instrument of writing, you can do pretty much anything that you want. The real question is, should you. If you accept that writing is a means of communication between writers and their readers, the question becomes, does a particular practice enable or degrade that communication.
Thus, if the specific set of readers that you are targeting buy into colons, use colons. But you have to ask if you might have more readers if you used more conventional punctuation. It is all too easy for the modern reader to click on the skip-this-whatever-it-is button and go on to something that is more familiar. One of the most important tasks for the writer is keeping the reader from clicking that button.
The ultimate answer to this and similar questions is, does this help or hinder the reader's experience.
