Telling your boss you're expecting. Help!!

How did you or how are you going to tell your boss that you're pregnant? I'm really nervous and anxious about doing so. I'm 16 weeks along and we are going into spring break this Friday so I'm telling them this week. (Tomorrow) Plus, a few people at work already know so I want to tell them before they hear it or suspect it. It's a pretty big school so I doubt they heard anything yet but I want to make sure I'm the one to tell them. I've been working in the district for two years but have recently taken a leave position. I've had a lot of positive feedback from superiors and there's talk about giving me a full time teaching position come Sept. Only problem is I'm due in Sept and plan on taking off from then till January. I know my career will have to wait unfortunately but I'm still terrified in actually taking the plunge and letting them know. I still get maternity leave and a secured position when I return (my job I had before the leave, as an assistant) but I know they won't consider me now for the better paying teacher position. And being that there are so many people that apply and want these positions there's nothing to say that they didn't give me the position bc I'm pregnant. It would be impossible to prove. I also just want to do the right thing and be 100% upfront early on ...so not telling them until the end of the year (when contracts come out) when I'm huge and 7 months pregnant is not what I want to do. Anyone have advice??