What will happen?

Lwiddis wrote:
"We would turn them away, and I know of many other parks that would turn them away as well."

Be careful denying accommodations to anyone able to pay the charge. One of those kids could be a budding lawyer interested in federal lawsuits and say the reason was their age.

Actually, 20 somethings are not a protected class. Pretending they were actually a protected class, I could present plenty of reasons their schoolie is not acceptable. I would go with we only accept RVs with RVIA certification. We also can hang our hats on our standard "we reserve the right to refuse service to anyone, for any reason". Then we could also prove we allow plenty of 20 somethings (and blacks, Jews, Catholics, Mormons, Chinese, Dutch, Korean, Hindi, and every other race, creed and color) to stay, The only discriminating factor is they are driving and staying in a school bus, not in an RV.
Filing a discrimination lawsuit isn't as easy as you make it out to be. First, they would need to file in the proper jurisdiction, which would be Federal Court in Montana. That will cost them money. They would have to hire an attorney and have their case survive our attorney's motion to dismiss the case. Next would come actually proving they were denied accommodations due to their age. That would be difficult, since we would obviously argue and prove we turned them away due to the fact they didn't actually have an RV, and we specifically require the all guest stay in either RVs or Cabins. We specifically prohibit sleeping in cars, vans or other non RVs.
Maybe they can overcome all that and win. Then they will have to prove damages, which would be in the tens of dollars. So even if they win treble damages, they would maybe win a hundred bucks. I do not shake in my boots when I turn away a schoolie or some other home built junker no matter what the age of the occupants may be.