Do I want the entire staff to know, or only a few key people?
When you can, take some time to think about who you are in terms of your personality and style. Do you generally prefer to confide in one or two friends, or are you more comfortable having many people to whom you can turn?
For elementary and middle school children, the primary teacher would probably be the most appropriate person to tell, at the least (and you can always enlarge your circle later.) For high school age children, the homeroom teacher or school counselor may be the most appropriate person.
Thinking about how you have responded in other situations might help you make this decision. You might also consider whether you want to speak directly to each person you want to inform, or if you would like your key contact to pass on the news.