I had a similar thing happen last summer, except that it wasn't a school bus, it was a police car, and I had no problem catching up to him after he'd forced the cop car onto the shoulder. Or, rather, the cop had no trouble catching up to him, and then I had no trouble catching up, too.

The offending driver passed me again a few miles on, a bit more carefully. I was relieved - I could see him taking out his frustration about the ticket on me when there was no cop around to see.

I don't know what it is about some car drivers that make them unable to stand being behind a bike even for the few seconds it would take for the oncoming traffic to clear so they could pass safely.