Op
I tried this , and it had some minimum success. I broke the team into groups based on ability. It served several purposes. first, it allowed the faster kids to do their workout and not have to wait for anyone. It also allowed the younger kids an opportunity to prove themselves and hope to get a chance to run with the faster group. And, to your dilemma, it allowed me as a coach to have those kids that tend to drop out all within one group. Therefore, it would not effect the team's overall workout. And, it gave me a chance to have a way to deal with the few that were, for lack of a better term, weenies. Some boys just were weenies, and didn't want to do a workout or didn't want to run the 8 miler. And some girls were just constant whiners. How did this effect them? Not so much when they were freshmen. But, when they were sophomores, juniors, or even seniors, it was somewhat of a slight to be put with the injured or slower group. Believe me, some whiners and weenies surprised themselves by working hard to stay up rather than be subjected to going over to that "other" group.
I made it point to never make the freshmen or slower runners feel like they were less a part of the team. I always encouraged them to improve at their own pace, and even told them about this or that senior that was slower than them, and that same referred runner is now a varsity runner who is considered a solid contributor.
Do what you feel is best. But remember, that someday these boys and girls will be grown men and women. You don't want them to have bad memories of you when they see you 13 or 15 years from now. Let them know you care, and that they are very good at ____________ which is much more important.
I had a few former runners who went on to be pilots, doctors, lawyers, and many would mention the support I gave them as a confidence builder, compared to other experiences where coaches ignored them or gave negative remarks.
Be a positive part of their lives.