The problem is deeply structural.
("why don't they just have the sports teams do one short practice in the morning, and one short practice in the afternoon?" 1. some of them actually do that already, despite the crazy-early school start time, and 2. for the less-insanely-dedicated sports that can't rely on parents to drop all the team members off before school, you'd need to run extra busses just for the sports kids, if they're showing up earlier than everyone else, and 3. Depending on the sport, one long practice is better and/or a lot more convenient than two short ones)
Another reason is that most older kids don't want to be covered in sweat for their entire high school career and having kids communally shower at school is seemingly something that is rightfully being phased out.
For a sports team, that seems silly.
Most sports have enough weird contact that the showers are probably the least of the problems.
Now, if you want to argue that sports, in general, and school should be separate, I'm all ears.
That said, there'd be nothing to prevent a school district from adjusting both school times if local parents demanded this be accommodated.
> In California there are 560 Elementary districts, 87 High School districts, 330 Unified districts.
(I do think that this isn't a great argument against switching to DST permanently. I think if this was legislated then the school districts would just adapt now, especially since a significant chunk of students are still doing remote learning.)
Small kids have few classes and before/after optional program that basically do childcare for parents. They are free to go much sooner. Many of them come in and out by public transport too.