Do you give kids target times for a workout or just let them figure it out? My high school coach never gave times. Everything was supposed to be as hard as you could go and finish the workout, which usually meant running the first rep too hard. Once I started actually getting goal times for workouts, I got a lot better at pacing.
You could also try calling them back at 200m until they get the first rep right. Usually, if they get the first rep right, they won't speed up drastically.
If you have groups that run together, you can usually figure out what kid tends to get out too hard. Tell them they're not allowed to lead in the first half of the workout.
Finally, let them pace badly for a while. If they're doing the same thing week after week, it's time to step in. If they're still improving, especially if they're an underclassman, let them keep doing what they're doing.