This is brand new - seems you now need to sign up for the high schools fairs (thanks Monica for the head's up!)
You can sign up for both citywide and borough fairs here:
I get that they're hoping to deal with crowd control issues. I've been in hallways that were completely impassable - it gets that overwhelming. But I can't imagine how they'll check in the tens of thousands of people who show up at these - the people staffing these events are already stretched pretty thin. Or, how everyone who's planning on going knows to sign up. But, it's an effort to address problems and that's cool.
I'll send more about how to deal with the fairs when they're a bit closer.