That depends on the direction you're traveling and whether you're driving in Washington or Oregon. Here are two common situations:
You're behind the bus, driving on a three-lane road (one lane in each direction, plus a center turn lane). Regardless of the number of lanes, you always need to stop behind the bus when its red lights flash. Stay stopped until the driver turns them off. Never attempt to pass on the right, where you could hit kids entering or exiting the bus.
You're driving toward the bus on the same three-lane road. In Washington, you can keep going as long as the road has at least three lanes or is separated by a median or a physical barrier. In Oregon, however, you must stop unless you're on a truly divided highway separated by an unpaved median or physical barrier. In that case, you must stop before your bumper aligns with the front of the bus and remain stopped until the red lights quit flashing. Courtesy of PEMCO