School Buses and other buses are required by law for safety reasons to stop at any railroad crossing whether there's a train coming or not.
So even if the railroad crossing lights are not flashing or the railroad crossing arms are not down or coming down the school bus must stop at the railroad crossing.
The school bus driver also opens the door to allow them to listen and hear better and to see further down the railroad track in case a train is coming.
You can always pass the school bus legally as long as it's safe to do so at the railroad crossing but you should also be aware of any potential oncoming traffic or trains.