My son was hit in the head by a lock from a passing school bus. Can I not only sue the kid that did it but also, the school for lack of supervision?

Lawsuits against governmental entities, including schools, are difficult, because governmental units are generally immune from lawsuits, with limited exceptions.  In a case like this, in order to be successful, you would have to prove that the injury 'arose from' the use of a motor vehicle - in this case, the bus.  As you can imagine, there have been many injuries that have occurred in and around school busses, for a number of different reasons.  In a case like this where the bus actually was in operation, the central inquiry is whether the injury occurred as a result of the bus's use or whether it occurred as a result of lack of supervision of the driver.  From the facts you presented, it appears the injury occurred due to   the bus driver's lack of supervision of the students, rather than an improper of negligent driving of the bus.  I do not think that you will be able to hold the school distyrict liable for this injury.  You would however, have a claim against the child that threw the lock.