In this case, there is a rule against it, from the BeachLax rules page...

Placement on a grade based team (2018AA, 2019AA, 2020AA and 2021AA) is dictated by the highest grade of any player on the team.
Any team in violation of the above roster rules will forfeit all games in which the ineligible player has participated and forfeit any opportunity to advance to the playoffs.

Given that there were documented current 8th graders on the Crabs team they were clearly in violation. What say you Aloha?!?