ANNOUCEMENT

Announcement is something said, written, or printed to make known what has happened or (more often) what will happen
In writing an announcement, keep the following points :
1. The title
2. Date/time, place
3. Contact Person

Example :
Attention please...
Mr. Sanjaya, our headmaster.
Will say some thing about the national examination
All student of grade XII are required to gather in the school hall 
After the second break.
Thank You.

formal announcement

informal announcement