Hitting the guy in the back would work. He's really only trying to knock him off his feet.
The scenario is that "Victim" is out running the school track. (It's a live-in school, but a bit more complicated than that.) Victim sees "Chaser" looming nearby, realizes there's no one else close, and charges off toward the only place that's going to have people, which is the ball diamond because "Pitcher" likes to practice there when no one is around.
When Victim leaves the track, hoping that the guy isn't really anything but someone lurking around, Chaser follows.
Victim runs screaming for help. Pitcher turns to see what's going on, realizes that Victim's being chased by this guy as they cut through the ball field and tries to at least trip him so Victim can get away (and him, too, as he has no desire to stay out there with Chaser).
A solid hit to the back could make him stumble.
An alternative scenario would be for there to be a pitching machine for batting practice, which I would assume makes for a more accurate delivery system if turned the right way.