Very simply, when a school bus stops on a four-lane road without a raised divider, all lanes of traffic must stop. If there is a raised divider on the road, traffic traveling the opposite direction on the other side of the divider does not need to stop.
The raised divider could be a grass divider, a concrete divider or any other combination you can think of. If there is no raised divider, and just perhaps a turn lane, all traffic must stop on all lanes.
It's a very important, yet often misunderstood law. With school back in session around the metro area, I think it's important to remind drivers who are out sharing the roads with school buses and sometimes forgetful third graders who really wanted to see Mike Mori on the other side of the street.