Having been in the school busing industry for almost 25 years, I have seen great improvements in most cases, of maintenance of buses. I do want to say though that an out of service can be for something as little as a single screw missing on the floor or a light that has gone out on the way into the shop. It isn't always a major crisis and can be fixed immediately so the bus goes right back into the line.
Most Mechanics are excellent and do care about their work. Our children really are safer in most districts than it sounds.