I am a retired Senior Social Worker and have worked with the police conducting video interviews with children who may have been abused. I agree curiosity is normal, but touching other's genitals is not. A simple explanation about some parts of the body are private and are kept to ourselves and seen by our Mummies and Daddies.
It is certainly important to report these concerns to the head teacher, or Safeguarding Lead in the school. One would hope that a member of staff on duty at going home time would be aware of children hanging round toilets.