I don't think you've thought this through.
So let's go through the entire progression. For the non-US among us, let me define some language. Elementary School spans from Kindergarten (age 5/6) to Fifth Grade (age 10/11). Middle School spans from Sixth Grade (age 11/12 to Eight Grade (13/14). High School spans from Ninth Grade (age 14/15) to 12th Grade (17/18). In some areas there can be a bit of squish - some areas have Middle School for only Sixth and Seventh, or only Seventh and Eight grades, but this is the most common separations. It more or less aligns with "mostly pre-pubertal", "in the process of attaining reproductive maturity", and "fully reproductively capable but still maturing".
For school sports, most sports become both competitive and sex-separated in Middle School. There's some competition among Elementary schools for sports, but it's not as common and not as intense. Middle is where we begin having extracurricular school teams, with try-outs, and frequently with a distinction between Junior Varsity and Varsity teams. In Middle, there are still a lot of non-competitive intramural teams. By High school, nearly all of the school teams are competitive JV or V teams, and there are few intra-mural teams.
A note on the term "competitive" in this context. Intra-mural teams are essentially open to anyone who wants to play, with very few people being excluded. They still participate against each other as well as against other schools. But they aren't part of a Competitive Circuit, involving play-off rosters, increased levels of competition at regional/state/national levels, and significantly less focus on trophies or anything resembling a competitor ranking system.
So now that we've got the basics out of the way... What is the impact of allowing transgender identified males to participate in female sports in "non-elite" school sports ?
This means that in Middle School, some males will be on the female JV and V teams. For each male included on those teams, there will be a female who is EXCLUDED from those teams. Those females will then miss out on the opportunity to practice and grow their skills, as well as the opportunity to engage in competition against other schools and athletes. You might say this is no big deal, but I disagree.
Without that middle school participation, those excluded females will be unlikely to participate in high school sports. Some of that will be because they have lost interest because they have been denied the ability to compete in middle school. Some of it will be because without that middle school practice and competition, they will not have grown their skills sufficiently.
So then... exclusion of females from female sports in middle school in favor of allowing males to compete on the female teams, reduces access to high school participation for females. Here we've already got a loss of opportunity for females.
In high school, if males are allowed to participate on female teams, this impact gets amplified. In High school, those athletes are being scouted for college athletics as well as continuing to grow and hone their skills. For every male who participates on a female team in High school... those are females who are being excluded from consideration for College teams. They will not be scouted, they will not be monitored, and they lose the opportunity for athletic scholarships.
Now, we have excluded females from female sports in both Middle and High school. We have provided expanded opportunities for males - they can participate in both male and female sports, whereas females are inarguably uncompetitive in male sports because biology is real.
College is the prime location for entry into professional athletics.
By actively allowing males to exclude females from female sports in Middle School... you are reducing significant opportunities for females in professional sports.