Here's the story of Sal getting punched: It was fall and he and Miranda were walking home from school as usual. There's a garage on the way home they have to pass with rowdy boys, and also the Laughing Man.
Miranda was carrying her "Mysteries of Science" poster, all about theories of yawning. She thinks that it's just nature's "semipolite way of telling someone that they're boring everyone to death" (7.4).
The day before there had been a fight among the group of boys. One of them held another against a car and punched him repeatedly.
On this day, though, the boys are acting pretty normal, but just as Sal and Miranda pass the garage, one of the boys in an army green jacket walks over to them. BAM. He punches Sal hard in the stomach. Then he smacks him in the face. He looks at Miranda's "Mysteries of Science" poster, and then he walks away.
What?! Without any provocation? Yup.
Sal, of course, is doubled over. None of the other boys are hooting or whistling. They're only staring. One yells at the kid in the army coat, "What the hell was that?" (7.10). (We'd like to know as well.)
Miranda eventually gets Sal past the Laughing Man (who yells "smart kid"), but Sal won't let her in the apartment with him (7.14).
Miranda didn't know it then, but that day was the end of her friendship with Sal.