used to practice HR for a property management company in another state and yes, we allowed school buses to use our parking lots as bus stops with permission. So it might also depend on an agreement between the PMC and the school district. Honestly since the OP/driver would have no knowledge of this arrangement, other than seeing a bus drop off children in the parking lot, I'd suggest he/she follow the regular rules of bus signage/lights even in a private parking lot. Plus you never know when a child will dart out ESPECIALLY in a parking lot setting (which to me is MORE dangerous than a highway) towards a car that is not coming in an opposite direction.

Think of it this way. Bus is stopped has its lights on in a parking lot...you keep going, don't see and strike a child...do you really want to make the argument to a jury or judge that the bus lights/laws don't apply because the bus is not on a public street? (as said by a mom of grown kids....)