Roy Said:

When writing a business letter to several people...?

We Answered:

Dear Drs. Smith, Andrews and Brown;

Note - you don't need that comma before the word "and". The colon following the salutation is frightfully formal and stuffy ... you can replace this with a semi-colon ...

Virgil Said:

I am writing a business letter to to a high school principal regarding my school being invited to see a play?

We Answered:

I know its probably too late - but you would start with the thanks and that you enjoyed it. then comment on something specific you liked or how the kids reacted to it. Finish with a statement about how you wish to keep the two schools in contact for future events/projects.

It can be short, I am sure the principal is busy. Perhaps have some of the kids from your school sign it.