Bullying behavior exist also in the bullied: The quiet kid had to appease the bullies.
Train these roles often enough, both parties are on a set course for life.
It's clearly implied in my understanding that the cool kids pushed him over however, and that you expect they were to let the kid sit. The interpretation still depends on the environment, and the details, because it is a legitimate concern in principle, that one might not want to associate with another, and it's left open whether the kid really wanted to sit with them or accomodate them with preferable seating arrangements, or frankly would rather stand than associate with them. In small terms it may be actually quite fair if such distension is communicated quickly anyway--same way that you don't elaborate nearly enough to reach an agreement with your conclusion unless the agreement was established prior to the conclusion.