Aw I wouldn't wait until they notice! That is awkward for anyone! I told my middle schoolers and they were playing a game of hangman in their free time! They are children in emotional support and were shocked at first. Then the next day they came in and surprisingly asked me very appropriate questions (how long have you known, boy or girl, can you feel it kick, do you have names, etc.)
Then I told the senior highs in youth group, "Just to let you know, I am not awkwardly getting fat, I am pregnant. So we can talk about it and don't be weird not knowing if I'm pregnant or just getting fat." They thought it was hilarious! But this was a different setting than in a school.