You can have your announcement to be on air for free, Since you are part of a non-profit organization.
Here is a sample e-mail,
I am from Harmony Science Academy - (Where you are). We are a charter school (non-profit) and I would like to ask If you can help us to announce one of our event that we are having on (Festival date). The event is called (City) STEM festival 2014. This event will be open to public. Currently Harmony students are working on their PBL (Project base learning) projects and, on the festival day they are going to present it to guests. You can check this photo album (You may link your last year`s photo album) for last years festival to have some idea about the festival. There will be raffles, science games, food and much more. (You may include more activities)
Festival is from 11.00 am - 2.00 pm (Time), (Date) Saturday
and the location is,