There is almost never an "overage" because there is a driver assigned per route, but they also have a bunch of sub drivers who get paid to show up, wait, and maybe take a route whose driver didn't show up, but if not, they go home after getting paid for an hour of waiting.

That's the gist, but it's a bit more complicated than that; in the school district where I live, bus drivers who were assigned routes actually had two or three per day: one for an elementary school, one for a middle school, and one for a high school. Yes, they are that short-staffed. In fact, all of the buses are advertising paid training because they need drivers that bad.

When I quit on my second day because of the stress about being pushed to get kids to school on time rather than safely, and also feeling like I needed more training to be safe, the head of the transportation department desperately tried to get me to stay.

tl;dr: they have bus drivers on standby who get paid to wait in case routes need to be filled. And they are so understaffed that overages never happen and drivers handle 2 or 3 routes.