The purpose of mandated reporting is to protect people from abuse. Mandated reporters tell child protection agencies or the police about situations that might be abusive. Mandated reporters must be at least 18 years of age. If you are under 18, your supervisor must make the report.
Most sexual activities between minors close in age, who both consent, are not reported. While there are laws against minors having sex, it is not your job to enforce those laws. Your job is to help protect people from abuse.