I am not very familiar with the way high school sports are organised in the US. I am all for some Physical Education, but I don't see why it needs to be all that competitive.
I suppose it might vary somewhat from state to state. I can only describe the Illinois system with any confidence.
First, high school sports are not Physical Education, and you don't get a grade for participating. However, at some schools, anyway, the P.E. requirement may be waived for a student during their sport season. When my daughter was on the softball team and the cheer squad, she had an extra study hall instead of P.E. (P.E. is also waived during marching band season.) In order to be eligible a student has to maintain a minimum grade point average, with grades reported every week.
The teams compete against teams from other schools in the area within classes based on school size. Larger schools are likely to have a deeper talent pool than small schools.
High school sports is the first "serious" level of competition in most sports. Arguably, it's the point when some athletes begin pursuing a professional career. Consequently, families have been known to relocate across the country so their kid can play on a particular high school's sports team. There are even boarding schools that specifically cater to sports training. For example, IMG Academy:
https://www.imgacademy.com/about-img-academy
Being high school, the teams for the public schools will be made up of a mix of serious athletes who are looking to del=velop and showcase skills to get a chance to play at the next level (college or pro) and more "social" athletes who are there for more casual reasons, often due to friendship with some of the more serious athletes on the team.
For some sports, like baseball, high schools sports are a direct feed to the pro level. MLB drafts students right out of high school as well as college. If drafted out of high school, a player who elects to play professionally, I think gets college paid for by MLB after their career is done.
High school sports are overseen by a state organization that sets eligibility rules and standardized game rules. In Illinois this is the IHSA. Obviously, the individual schools can add additional eligibility requirements on top of what the IHSA requires. IHSA is, as I understand it, a private organisation, but their power is somewhat limited by the fact that most of their member schools are public schools.
There are also club sports, which are independent private organizations. In these, the teams are usually organized by maximum age levels: 12 and under (12U), 14U, 16U, 18U, etc. Often, teams are made up of athletes who play against each other on high school teams. My experience with club softball was that the games sucked because they had time limits. We would go to tournaments where there were six to 8 games playing simultaneously in hour increments. So there might be fifty games at a tournament in a single day. Because of this, club sports are starting to replace high school sports from a college recruitment standpoint. A coach can go see many teams/players play in one stop.
But high school is still important as the high school season is at a different time of year from the club season.
Even with competition at high school level there is going to be much less of an issue since it would only require a rule to accommodate teenage transitioners. Unless they are banned from receiving the healthcare they need and forced to go through normal puberty, transgirls competitive advantage over other girls is going to be absolutely minimal.
That would depend a lot on the age that the transition began. High school runs from about age 14 to age 18. This is a period when there is a lot of development going on. If you look at a high school yearbook, the freshman and senior classes do not look like peer groups. So someone who begins hormone treatments at 16 will have already developed more than someone who started at 12. That's actually the same reason why some trans people advocate for hormone treatments (or at least puberty blockers) at the earliest possible point. (And, if the diagnosis is correct, they are right.)
Yes, because in the LondonJohn presumably lives in the UK, where there is no law banning men from using women's changing rooms. I don't think there is such a law anywhere in Europe, and despite attempts at creating them, there are no such laws in the US. I think there are in Saudi Arabia though.
As Meadmaker pointed out, while there are not specific laws, other laws apply.
What is banned is demanding to see proof of someone's sex before allowing them in women's changing rooms; a law instituted to protect women who look a bit butch from being harassed.
No, the loophole already exists. I don't think there has ever been a time when it didn't. If you want to close the loophole (requiring scrapping existing anti-discrimination laws) I think it is up to you to show that doing so will decrease harm to females.
Doesn't even need to claim being a transwoman. Just needs to act non-disruptively.
In the US, trespassing and voyeurism laws apply. Don't get me wrong, if you accidentally wander into the wrong bathroom, you aren't going to be arrested, assuming you turn around when you realize your mistake. But a property owner can have you given a trespass notice and ban you from the property if he thinks you did so purposefully.