In successful groups that I've been around (HS, middle, youth, and serious sub-elite adults), a few things stand out for me.
First is having some peer competition. If you have one runner that is far & away faster than the rest (e.g. 15:30 5k with everyone > 17:005k), nobody is there to push and challenge the top runner, and the rest of the pack is too far back to realistically think they can train like #1. If you have two top runners they push each other and there is always friendly rivalry. But the more friendly rivalries you have throughout the group, the more everyone benefits. This is one reason I think bigger schools are better. It's not just about having a larger talent pool, it's also about having a larger rivalry pool. Corollary to this is that bigger schools have a more complete continuum of runners. When you have large (e.g 2-minute) gaps between your training groups, it's tough to visualize jumping up to the next level. But if you have a ton of kids running, that runner just ahead of you is within striking distance, so there is motivation to train harder to catch the runner that's just a few seconds faster and move up. So big schools have lots of little stepping stone gaps between runners while smaller schools often have insurmountable chasms.
Second is having an example to follow. Running takes hard work, but until you see it and experience hard work in training, you really don't know the extent. If you are an 18:00 5k runner, and you want to be a 15:30 5k runner, seeing one (or more) practice everyday is extremely important. You can use that fast runner to visualize yourself being that fast. Other runners see that there is rarely any magic. It's hard work to be fast, and "talent" is secondary to hard work. Other runners know the extra summer mileage, they know about the core work and weight room, they know about the nutrition and good sleep habits, and they see the results in that #1 runner.
Down the line, faster runners inspire slower runners. Again the bigger your group the more this flows from top to bottom. 20:00 5k runners see 18:00 5k runners as fast, so they want to emulate them and can see that as a goal. Then when someone breaks through by following the example of success, it reinforces that as the path for improvement.