There is also MUCH more space in an open gym than there is on a typical stage in an auditorium. Unless you're willing to have gym fans circulating the air out into the audience, you're getting MUCH more recirculation of air and aerosols in a band setting than you are in athletics.
Oh - and that's 50 players in ONE ensemble. How many bands will perform at your concert? Two? Three? Four? How many beginners do you have? Can you distance all of them safely?
And also consider that 100% of your kids will likely be UNMASKED while playing those instruments and making that concert happen. You'll have 50 unmasked individuals in the same place at the same time (well, a few less if you can get your drummers to keep their masks on) as opposed to the 10-12 that are unmasked at any one time in athletics. That's at least four times as many potential exposures.
Don't even bring audience/fans into it - as a state mandate, audience members and fans are supposed to be distanced, masked, and at 50% or less capacity. If your district isn't doing that, that's on them, not you.
If you truly don't understand the difference, please, seriously, actually ATTEND a basketball game or a volleyball game before you throw metaphorical stones. Count the participants.
Please don't bring up "well, athletics gets to" as a reason to your admin because it's not an equal comparison. Given the data and research on how COVID spreads, our activity is MUCH LESS SAFE than sports and has a much higher potential to be a "super spreader" event if you insist upon holding a concert of this type, especially with the outbreak numbers the way they are.