A formal letter

Can you show me an example of formal letter.
If I was the manager of a new museum and I wanted to write to the headteachers of the local schools to encourage them to bring their pupils on a tour, what is the ltter going to look like

[Tdol]

It will look something like this: Formal Letter Writing - Articles - UsingEnglish.com
However, it is up to you to write the letter; we don't do people's assignments.