Private sports club in school tournaments?

Hi. As stated above, can private sports clubs/team (example: teens who are around the same age but goes to different schools) participate in middle/high school tournaments? Like, for baseball for elementary school kids, they have teams in the little league(even though the kids in said teams are from different schools). So if they were in the big leagues, can they participate in the high school Japanese tournament to go to Koshien?

I tried to find the answer on the net but no luck. So if anyone knows, please answer.

I'm not 100% sure but this is what I've seen.

Many students focus on one sport at school in MS/HS and compete in the school tournaments.

It's also common for very high performing sportsmen and women to be involved in sporting clubs that are involved in their own tournaments which are not linked or related to school sports.

One of my previous students was really into soccer but was only involved with his local Japanese team but did not get involved with soccer at my old school.

As far as my experience goes with JHS and what I've seen of HS, no, that's not how the school tournaments work. If you had a group from different schools and can choose the best of them all, then that's not fair to the teams where all they can draw from is their own student bodies. Koshien and the like is only for school teams, not little league type teams. However I think those kinds of teams have their own tournaments...

Sports leagues in high school dept are accepted by individual school basis, not a private club basis.

As stated above, can private sports clubs/team (example: teens who are around the same age but goes to different schools) participate in middle/high school tournaments?

No. School teams might possibly be able to enter a club tournament but club teams cannot enter the school tournaments.

So if they were in the big leagues, can they participate in the high school Japanese tournament to go to Koshien?

No, Koshien is a national tournament specifically for high school teams.