Jason Balkman ran in the 4:20's in 8th grade and went on to be a top notch college runner... So it is possible. As long as the kids are running that fast and aren't training too hard yet they will improve.
What everyone needs to remember is running there are only a few people who can run sub 27 min. 10k's or a 2:05 marathon in the whole world. A 4:30 miler for a 8th grader is great, but not winning gold is hardly a failure.