While right now not much high school specific stuff is going on, things will ramp up by the end of the month. Some schools will start posting info before schools starts. And some schools will host things just as the school year starts.
A bit of advice, while it's nice to get things out of the way early, too early could mean everyone's not necessarily in the back to school groove yet. I know Brooklyn Tech starts their tours second or so week of classes - tour guides are just learning their spiels then. I made the mistake with my older one, to sign up for the very first Bard testing session (they offer them through the fall). She wasn't into school mode yet and it didn't go well. Lesson learned.
Pacing is something to keep in mind. That thought can be easily lost as sign ups and open houses start happening. I'd suggest marking things in calendars so you can see what you've got going on and when, making it harder to over or double book. I managed to do plenty of that too.