Roy thought that fighting back against Dana Matherson would be a waste of time because Dana outweighed him by about fifty pounds. For his part, Roy distracted himself by watching a strange boy running on the sidewalk, parallel to the moving school bus.
According to the text, Dana had grabbed Roy's head from behind and rammed it into the window of the school bus. The bully had then held Roy's head against the window for good measure. Dana's goal, of course, was to earn the satisfaction of hearing that Roy had had enough of the pain he was being subjected to. For his part, Roy was more interested in why the strange boy wore no shoes, carried no backpack, and toted no books on what was obviously a school day.
Roy had hardly felt Dana's fingernails digging into his scalp, perhaps because his threshold for pain was greater than Dana had realized. So, Roy didn't fight back against Dana for two reasons. First, Dana was about fifty pounds heavier than Roy. Fighting back would have been "a complete waste of energy." Second, Dana was known to be the resident school bus bully and a "well-known idiot" who probably couldn't be reasoned with.