The names would have come out over the next few days, anyway. McGonagall’s choice was to either break the news to all the students on her terms, or to have wild rumors appear within hours.

Breaking the news herself gives her the chance to declare her solidarity with the affected students in the clearest possible terms and to quench any schadenfreude immediately. She is proactive, rather than reactive. In fact, compared to the Minerva McGonagall of the very early chapters, she feels a little more grown-up now, in a way. She has developed into a more sophisticated character over the course of the story, and I like this a lot.