First of all, do staggered starts, i.e. your training group should all start at different points depending on their speeds. Faster runners at the back doing the chasing, and slower people at the front try not to be caught. You should all finish around the same time.

Isolated drills, for technique and efficiency. Under pressure, many people lose their form and run even slower.

Winter training, this is where you do the stamina and endurance work which is very important for sprinting. Nobody can run 100m 100% flat out, everybody fades around to 60-70m point. With good edurance, you can reduce that fading. Have you ever seen athletes that look like they are accelerating towards the finish?
They are not accelerating, they are actually fading, but fading at a slower rate than everybody else so it looks like they are speeding up - strange right? Could endurance in the winter should help this.

Weight training, mainly in the winter. You should concentration on compound exercises such as squats, bench press, clean and jerks.