the parents of the kids who couldn't advance to state write the AD and principal and explain what happened. "coach x entered the y HS team in 10 regular season HS indoor track meets this season. state rule is the limit is 8. as a result no one from this HS can advance towards state. that includes my son, johnny, who was ranked 10th in the state in the 800m, and was qualified to advance. etc. etc."
stick to the facts. keep it tight on this objective mistake. don't wander onto other complaints . limit the emotional crap. you want it where the coach has zero excuse and the mistake doesn't disappear into a bunch of emotional bs that's more debatable. or worse, allows the coach to chat up how in the abstract, "they would have qualified 10 kids to advance." you want the focus on the reality that none of them can go anywhere.
make your point, get in, get out quickly. save the broader critique for when some other objective mistakes pile up. and by objective mistakes, no doubters like this, as opposed to debatable ones like "she has an ego" or "we don't like her training methods." she has an ego and doesn't admit her mistakes, goes better when she makes a mistake. we don't like her training methods, goes better if half the team is on crutches or can show they are running slower not faster.
protect the kids best you can by leaving the kids out of it. parents make the complaints. if the coach takes it out on the kids, voila, that's a new objective beef.
numbers show it's not just one parent with issues or a vendetta, but something affecting the team. so if you do an email approach, have 10 parents do them. if you have a meeting with someone, 10 parents show up.