At my daughter's primary school there is an unadopted road that leads up to the front gates. Every week there is an e-mail from the school saying "please do not park on this road". It is quite narrow, there are lots of kids about and parking there disrupts access to the school. They had a bit of a campaign asking parents to report cars parked there and the weekly e-mail did have a bit of a "shaming" section of registrations for a while.
So if you saw someone parking there would you "grass them up"? Or would it be a little petty and ultimately probably serve little purpose other that to make yourself feel a little smug?