To sum up what they've found:
1) Wind instruments do in fact produce aerosols. Trumpet and Oboe seem to be among the worst in the UC study. In the CSU study that's about 6 weeks behind them they're showing Oboe is more moderate. Flute surprisingly is showing very little production.
2) COVID-19 is an aerosol spread virus - that's been shown by multiple studies.
3) Bell Covers reduced aerosol transmission a lot!
4) The other piece of the puzzle that it sounds like you aren't doing is the slitted masks. I'll be the first to admit they're a pain in the rear to use, but the kids get used to them. They're effective because the kids noses are covered as they are playing.
5) Aerosols build up indoors over time and 30 minutes should really be our maximum time we are rehearsing before clearing a room.
6) Outside is significantly better.
7) Lining kids up in a straight block formation is the safest because aerosols would hit the back of your head, not the front.
You ask about rules we are legally following? TEA has reduced social distancing guidelines and UIL has put the loophole 'when possible' on everything. Legally you don't have to do anything. Morally is another question. Personally I'm taking every safety precaution we can so that when we have cases in our school I don't look back and trace a massive outbreak to the band. Like I tell my kids, when it happens I don't want it spreading.
Is this going to fix when they're huddled up without masks on in Volleyball or Football? When you have people wearing masks below their noses? Just wearing gators (pretty much ineffective)? No it's not going to. But I can say at the end of the day that in my little corner of the world I did the right thing and as much as I possibly could to keep my kids, staff, and community safe.