Looks like you just say you are from Quincy and you get in.
"Looks like you just say you are from Quincy and you get in" is an answer, but not what I would expect from those on this forum.
As someone who was denied access to watching a game in person last year, here is a way this can successfully work.
1) Players/cheerers/etc. from Alton can be given physical tickets to attend the game. And then at the gate, they show the ticket and proof of who they are. People at the gate will have a sheet with names of Alton people allowed into the stadium. If you don't have a ticket and can't prove who you are, you cannot get in from the Alton side.
As for the Quincy side and "fake" Quincy people who are really from Alton, there are two choices:
1) Quincy sells tickets to the football game at their own school on Friday and if you can't prove you are from Quincy in the athletic office, they don't take your money and you don't get a ticket.
Or, 2) At the gate at Alton, someone without an Alton ticket has to prove that he/she is from Quincy or does not get in.
If you think that is a slow process at the stadium gate, well this is obviously a depressing situation and every person attending will know that it's not going to be a quick entrance into the stadium. Plus, it's not. going to be a very big turnout.
The Alton program has collapsed. They are 3-22 since the 2020 season (1-5, 1-8, 1-8, 0-1) and its football coach quit a week before last Friday's season opener.
As for my disaster, I was not aware that I could not attend the game as a regular "fan."
I waited in a line to get into the game and when it was my turn, I had to show that I had a ticket from the "home" team or a ticket from the "visiting" team.
I was just a fan wanting to watch a game, and had neither. They said I could not get in, and I said it was a shame that it wasn't publicized beforehand, although quite frankly I would have had to look at a school website to find that out. I mean, the game date and time I already knew so there was no need to look anything up.
So yes, it's doable and my impression is that people will understand the unusual attendance circumstances.
Besides, it's like a long drive from Quincy to Alton, so nobody from Quincy is going to go to the game without being sure of getting in, and who from Alton is going to make like a four-plus hour roundtrip to drive to Quincy High if that's where you have to go to buy a ticket.