I recently downloaded a lot and moved my family in there.First of all,even if the lot is an apartmentbase(in the neighborhood screen there are shown 4 apartments which are included within the lot),when you enter the lot,and go to its information,it says : Zone Type:residential.You can also build new walls,windows,doors,and so on(things you're not allowed to do on an apartment base)
It's been two days since the school bus didn't show up,and I'm really scared!

I have no idea if the zoning is affecting the school bus, but if it has always shown as residential you might try the "ChangeLotZoning apartment" cheat. And then notify the creator.

Do school buses show up at any other apartment lots that are properly zoned? If not, you have a more serious problem.

Either way, back up the neighborhood while it is still like this. Backups are handy. Especially when the social worker takes all the kids away if one gets an F.