Actually Italian, your very wise post says something much much more important than mine did
It IS about that transition from it being purely your journey, into your childs as well/predominantly. When you're going through the approval process, it can feel very natural to tell lots of people, or your BC to tell their class...because at that moment in time, the future adoptive child is an abstract idea really...sometimes it feels more real than others, but nevertheless, the future child is a hope, a dream, ideas...not a solid little being you feel enormous responsibility for.
So it is, as you said, basically purely your journey until you get matched. Then suddenly this child is real. You truly start to comprehend what would be right and what would be wrong to tell people, you start to think "what will x think about this when he/she is older"
And then they move in, and suddenly it's all about them, as it should be. Their story, their privacy, and for the first time having to reconsider the openness that might have been very natural when you perhaps told people lots of details about the approvals process. Suddenly the fact that your BC told all their classmates feels uncomfortable. Because it's not all about their excitement any more, it's about the fact that if any of the tell others, your AC has lost the right to decide for themselves if their classmates know
It's a bit more complicated for older children who might start school mid year etc and have less choice about privacy, but similar principles
Anyway, I'm just repeating you, so to sum up the entire post in 3 words....what Italian said 