I suppose individual stakes could allow different things for their own regular-season games, but at the region level, the rule is clear: if a kid makes their high school team (grades 9-12), even if they don't ever see the court for a live game, and even if the HS season is over, they are not eligible to play church ball in the regional competition.

I was the stake sports directory for 4 years in Utah County, and every year I had coaches whining and moaning about it or telling me I was wrong or just playing high school players and hoping that nobody would notice.

I would suggest that you contact your stake sports director to clarify what the policy is for regular-season and regional games.