As a secondary teacher I have tried to implement a quiet hand raising method to get the class' attention, but this never works unless some verbal cue goes along with it. Therefore with a clear teacher voice, I say, "Ladies and gentlemen, can I please have your attention?" while raising my hand. Practically every time I do this action, all students listen or get their classmates to listen to me as well. It is not a perfect way to get their attention, but it is the most effective.