How do you win a traffic trial not stopped for school bus?

Tell them that the stop sign on the side wasn't popped out. They will let you go scot free.

Depending on the state, no stopping for a school bus is a fairly serious traffic offense. Some judge's may let you go if you can prove that the stop sign was not out, but generally, the presence of the school bus stopped on the road should be enough to give the driver notice to stop.

Additionally, even if you say the sign wasn't out, the officer will likely testify that it was actually out. In the case of your Word (with a reason to lie) vs. the officer (with no dog in the fight) it is rare that a judge is going to believe your side of the story.