I'm not surprised the police called it prank. Let me tell you a little story of my childhood for a second. I was bullied, yes even by girls, a lot. One day me and my younger brother and older sister are going for a stroll at the park near the school. When some of the kids who bullied me were around and yes they were girls. I was alone at the time, my sister was attending to my brother who fell down. Well the girls stop verbally assaulting me, yelling at me, calling me names. I'm going to ignore them and walk away. But they follow me and my siblings. I mean they literally were following us around and they get another boy to come over as well. They start yelling at us calling us chickens. Finally the boy comes up and pushes me from behind. Told me to fight him. Well I'm just going to walk off. Instead he punches me in the face. I got a tooth chipped and a bloody nose. And the girls are still harassing us until we get back into the neighborhood. We know were these girls live and we know their address, we also knew who they were. Well my sister calls the police. The police arrive and all the kids are around. We take our statements.
And the police tell the parents, "We do not know which one is lying". I was the one covered in blood here, with a chipped tooth still have the chip too.None of them were touched and we wanted to ignore them.
So I'm actually not surprised that the police say it looks like a prank. Absolute power corrupts absolutely.
To the topic at hand, I do think the girls should get some form of punishment. Be it vigilante or be it an actual police action. I believe in vigilante justice when its called for and in this case I think its called for. I think that is if the mom wanted to.
One more thing about the police. The police tend to not take younger folks seriously. Because they are kids, they may discriminate against age. I do not think these girls should get away scot free.
They do not have any consequences towards their actions, thus they'll do it again.