I think that is the exact right thing to do. I feel it should be up to the parents to decide if they want their children to know something like that, no one else.
My sons attend the same school. I told my oldest what happened at Sandy Hook before anyone else did. He's 10. He took it well. I did not tell my youngest what happened. He's 6. He would not have understood. I made that call, and it should be my right to make those calls, no one else. Handled it the same way with today's tragedy, in fact.
My sons' school also did not tell the children about the 10th anniversary of 911. I appreciated that they did not. When I asked my oldest son's teacher if they would be talking to the kids about what happened, she told me no, that that should be the parents' decision, not theirs, and I totally respect this school for giving me that right.
We parents know best what is right for our children before anyone else does.