I think half marathon for your absolute studs, the guys looking to run 16:00 or faster and the girls looking to run 18:30 or faster, 10-12 for your varsity/top JV athletes that want to earn a varsity spot, and 8-10 for the rest.
Peak long runs for 5k should be an easy aerobic to moderate effort though, you want to reap peak capillary growth and the slow twitch muscle gains without destroying your athletes for the next 3-4 days.