Different sports have different deadlines, but in general, an athelete competing in a 'club' sport in the off-season cannot play for his HS coach. An incoming 9th grader is included. But there are 'windows' of time where it is permissible.
In baseball, the HS coach can work with his team all he wants up until Aug. 1st., including tournaments, travel ball, etc. After Aug. 1st, if the team is still playing, he has to turn it over to someone not directly affiliated with the HS program ... which is why he'll usually have a non-HS co-coach that can carry on with the team.
This is why you often see HS coaches that are involved with off-season club teams moving any players from their own school to other teams coached by other HS coaches ... usually someone they know and trust as a friend that is coaching in a different league.