I remember seeing it in grade school and it was old then.. 60's or 70's. A clock would announce what time it was and what you were then supposed to do. ie: "12 o'clock time to eat, Time to eat" and it revolved around kids playing this "new game" they were worshiping someone... the adults eventually discover that kids all over the world are now playing this game and they all started playing it at the same time. Then all the adults are killed. I hope that's enough info to go on.
The story you're thinking of is called "Zero Hour" by Ray Bradbury. It was published in his 1951 short story collection "The Illustrated Man". I do not believe it was included in the movie of the same name derived from several of the stories.
It was made into a television episode for "Ray Bradbury Theater" airing Jan. 10, 1992. Watch it here.