I am a teacher and, even though I don't get to hear the heartbeat until next week, I really wanted to tell my seniors before they graduate. I had planned to tell my administration today and the kids tomorrow, but there was a moment in my first class that was an easy lead in this morning so I told them. Great reaction! Three of the five classes applauded and they were all very sweet.
Unfortunately, I couldn't get in to tell my admin as I planned, so hopefully word doesn't spread too fast.
Re: Telling Work
I've contemplated this situation too, should students have found out before admin. If admin finds out before you tell them, you should just say that a student asked and you turned bright red, couldn't lie, it was pretty obvious you were, and you couldn't deny it. Otherwise, get into an admin's office first thing tomorrow morning and let him/her know! No big deal.
Good luck!