Not a bus driver but I’ve seen enough of them to answer this question.

When they aren’t driving the bus, they are either waiting for kids to get on or off the bus or they are briefly investigating who threw the stray projectile that hit them before they pulled over but usually settle for the complete silence as each kid pretends to not remember what throwing even is, let alone admit who threw anything. That silence never lasts more than a minute after they start driving again, though sometimes it will only be whispers for a while if the bus driver screamed in rage or broke down in tears.