Many 6th grader girls are actually faster than 8th graders. When you are an 8th grader, it is demoralizing to lose to a 6th grader and that may cause the 8th grader to loose interest in running. So, in a way, I agree, it is unfair to those for whom the competition was intended.
On the other hand, the rule was likely arbitrary, introduced by some axx who probably claimed that he has seen the perils of running at a young age ... right. There are worse things in this world than having a kid who wants to run participate in a running competition. Hopefully, OH will see that and find a way to engage the younger runners.
The crime, if that's what you want to call it, is that what they did was against a rule. Clearly there are those who disagree with the rule. Either they get caught and pay the price or enough others will take the hint, brake the rule en-masse which would lead to a rule change. Isn't that how we do things?