I think a bike can stop quicker than a big rig, so you're probably safe as long as you keep very attentive, and remember that you aren't visible to the driver. No need to ride with the wheel on the bumper, give them a good 10 feet or so.
I remember as a gradeschool kid, drafting my own schoolbus. But, still learning the ins and outs of the toeclips.
The bus stopped. I stopped. Feet remained in the clips... and you know the story, and half the school watched me do the classic flop.