Very rudimentary maths, I'll give you that.
Numbers say there are about 300.000 transgender kids in the US, lets assume 150.000 transgender girls. There are 42.000 high schools in the US, so lets round it to 4 transgender girls per high school. Even if they all wanted to play the same sport, which is obviously irrealistic, it wouldn't be enough to start a team. So, in practice, they would be excluded from sports.
Let's say it' s a solo sport, like running. What are the odds that more than one girl would want to practice it per school? Very low, so is the school going to allocate resources, like a coach and travels to competitions for just one kid? In an ideal world, sure, in practice, this hypothetical girl would be excluded from sports.