When this happened to me I was a little kid. I got so embarrassed when some mean kids pointed it out to everyone and tried to play it off by saying I left it like that on purpose. For air conditioning..... I feel for you OP but hopefully you handled it better than I did lol
I personally don't get why it's such a big deal if your fly is unzipped. It's not like your body parts are gonna magically appear through a one inch zipper.
Just remember that if people noticed they tried to ignore it. Yes it sucks, but chances are that anyone who noticed it didn't continue to stare at it and think about how it was down.
Happened to me 5 times when I was in college.
Of course, the last time it happened I also had a chubby.