Holling had to go to the emergency room to have his butt x-rayed.  

Toward the end of the January chapter, Holling has to walk home after a very long day at school.  The weather has been really bad, and the roads are completely iced over.  As Holling comes to an intersection, he sees two things.  The first is the school bus sliding out of control across the intersection.  The second thing that Holling sees is his sister walking directly in the path of the out of control school bus.  Holling doesn't hesitate.  He takes off running straight for his sister and is able to knock her out of the way just in time.  Unfortunately, Holling is unable to get himself clear, and the bus bumper hits him on the butt.  

. . . which I saw from a sort of aerial view, because the right rear bumper of the bus caught me where I had worn my white feathers . . .

Holling, Heather, Mrs. Baker, and Mr. Guareschi all go to the emergency room to make sure Holling hasn't broken anything.  

The nurse came for me soon afterward, and my buttocks were x-rayed. . .