The Skool is the information center that Zim chooses to exploit. It's also where he runs into Dib, so maybe that wasn't a great idea.
There are many locations in and around the Skool that are shown in Invader Zim, including the playground, the cafeteria and Ms. Bitters' classroom. Because the Skool is used frequently as a setting, we get to see quite a bit of it in Invader Zim - and not just the normal parts. In Lice we get to see the cavernous grottoes that lie under the Skool building, in Dark Harvest we get to see Zim make use of the ventilation system to move unnoticed throughout the hallways on his nefarious evil trips, and in Bolognius Maximus we get to see the Men's room which was just used by Willy.