Continuity mistake: When they guys are fleeing the house in the woods, in the long shot Meat is in front of Mickey, but in the close up Mickey is in front of Meat. (00:21:25)
Continuity mistake: When Porky pulls the lever we see the guys fall into the river. Then from another shot we see them get out, and go round to argue with Porky, but they are now dry. Impossible considering the few seconds time difference. (00:38:05)
Continuity mistake: When Brian Schwartz drives away in his sports car, you can hear him shift 4 times. When they show a shot of his car go by, he is only going about 5 miles per hour. (00:48:40)
Continuity mistake: When Timmy cleans out his gym locker, he took only two items from the locker, stuffing them in the basket haphazardly. In the next shot when he is at the counter returning them to the phys ed coach, the basket is now full of at least six different items, all neatly folded. (00:52:45)
Continuity mistake: Coach Brakett (Boyd Gaines) finally convinces the young gym teacher, Honeywell (Kim Cattrall), to go with him to the boys' locker room. Once they are up there he disrobes her and gets on top of her. When they cut to a close-up, you can tell that he has longer hair than just two seconds before. (00:58:40 - 00:59:40)
Answer: He likely heard about it around town. Anti-semitism was fairly common at the time so word of someone being beaten in a fight by a Jew would travel fast.
So then, how did he also know about the incident with the black man? It happened clear out of town and the only ones who knew about it were Tommy and Billy since they set the whole thing up and the others didn't know about it until after they run out of the hut.
The rest had been back for a while before his dad showed up. Back then this was a small town and again word gets around real fast.