While I've enjoyed all of these events, some visits were definitely more valuable for the participants than others. I'm convinced that the difference between a successful author visit and one that wastes everyone's time and money is a little advance preparation by the school's teachers, students, and parents. Not every author is as good at presenting their material in person as they are at writing or illustrating.
Some authors are better at handling big groups.
Some are better with small groups. Some want to talk about their latest book. Some want to talk about the process of writing or illustrating. The best way to make both you and the author happy is to spend a little time and effort searching for the right author for your particular needs. Make sure that you are as comfortable with the author's presentation style as you are with the author's books.