Congratulations on your wonderful news! I teach first and am expecting my first child in early June. I had the best time telling my kids... I did it during a modeled writing lesson in which I was writing about a big surprise that I had to tell them. I thought my kids were going to explode, they were so excited. They've really latched onto the pregnancy idea. They see it as their job to take care of me, and they actually stay quieter now because they don't want to scare the baby. I even brought my ultrasound pics in to show them, and they were so interested. I was worried about being pregant at school, but it's turned out to be the greatest thing in the world for me and my class.
I hope you have as much fun with yours as I am having with mine!