The reason those troublemakers are laughing is because they told you "nothing happened"...

The reason those girls who caused the trouble are laughing as they walk out is because the police officer emphasized to the female victim, "Nothing happened."
But now, they are forcing her to turn herself in, right? If they do that, in principle, something will happen to you all. No one will marry you.
The police officers lied to you. They even mocked your parents.
You suddenly became anxious, didn't you? Go ahead and inquire at the police station. This is a tactic used by those officers.
If you undergo a police investigation, in the end, no one in the Republic of Korea will marry you. Just consider your criminal allegations. Police officers cannot, and if a problem arises within the organization, they will try to investigate it in principle.
Ultimately, the police officers viewed you merely as subjects for case processing. In reality, the problem lies within their own organization.