What had they been doing previously to get to 18:00-19:00? I think it would be helpful for people to know that first before they suggest where to make improvements. It would also help to know if these runners had plateaued under their old training, or are these kids who ran 18:00 as juniors after running 19+ as sophomores, and could conceivably drop to low 17:00-high 16:00 just by sticking with the same plan and gaining another year of training and racing experience.
Track workouts are all well and good, but make sure you arrange some workouts on trails, either fartleks, or measure some 1 km or half mile stretches of trail with a wheel to do some intervals- I figure it's more relevant to XC to practice that way, and I always thought running through the woods was more fun if you want to keep the kids interested.
One other thing that helped me as a high school runner was doing a couple 800's at the target race pace the day before the meet- this helped me remember what race pace should feel like and avoid getting pulled along too fast at the start. If they're like a lot of young runners, they might have split that 18:00 5K with a 5:20 first mile and 6:00+ last mile or something- teach them to go out at a controlled pace and I wager it'll be worth a 20-30s improvement even before you work on their fitness.