If the school bus is stopped at the railroad tracks and you still want to pass the school bus while it's stopped at the railroad tracks you can do so legally as long as it's safe to do so.
When planning to pass a school bus stopped at a railroad track always be alert and make sure there is no train coming or other vehicles in front of you to prevent a crash.
School Buses must legally stop at the railroad tracks whether the crossing is down or not and even regular drivers should look to be sure there's no train coming just in case the lights do not work or the railroad crossing arms fail to operate.
But you don't have to wait behind the school bus at the railroad tracks if you do not want too and you can legally pass it.