I’m not sure we can say for certain that teenage girls are crueler. It depends on whether a whispering campaign is “crueler” than a bloody nose. A boy who was tormented by the class bully throughout high school is likely to think his treatment was worse than that suffered by a girl who was “merely” the subject of insidious rumors.
Traditionally, teenage boys engage in physical confrontations. There’s a challenge, a fight, somebody loses and the fight is over. The pecking order is established and will not change. Teenage girls have a tendency to get caught up in drama that doesn’t have as neat a finish. It can go on forever, along with the pain it creates.
We also have to consider the fact that there are fewer potential aggressors in physical warfare than in psychological battles. Not all the boys in a class will be strong enough to launch a physical attack, but all the girls can engage in an emotional one. Weak boys don’t try to beat strong boys, but weak girls can undermine stronger ones.
As with everything in life, the big issue is how the recipient of abuse responds to the attack, whether physical or emotional. All of us get knocked down, but not all of us stay down.