I understand the dilemma. There are many sides to this. For instance. Two grown-ups are talking and one child takes something from another or gives them a little annoying tap or push. It's not meant to hurt and the child pushed or tapped is not crying. But they very loudly protest and interrupt the grown ups and I believe that's pretty much what the child that did the pushing or tapping meant to do.
I for one have a huge beef with people that allow kids to constantly interrupt. I think it's rude and that kids need to learn their place in the hierarchy of life. We give them so much of ourselves all day long. We cook for them, clean for them, change our lives and plans for them, entertain them, and create parties, experiences, and all kinds of nice scenarios for them. They sure as heck can learn NOT to interrup. And yet...how to teach a kid when it's right to do it? I tell them if someone is being really hurt... They are bleeding, crying, knocked down, or in pain, then a grown up needs to know NOW. We can't always be right there, and sometimes even when we are right there, stuff still happens.
If the child was scratched as in the skin is broken, then they NEED to tell the teacher NOW. That is not acceptable and I would not see it as one little scratch. Hurting someone is not allowed PERIOD. But, if it didn't break the skin, then maybe the other child was just trying to annoy him or get his attention or make him holler out. It's never too early for a child to learn that other children will try and get them into trouble.
In my world of 24 hour daycare, this subject is very difficult for me. I want to instill good morals and ethics into these children. I have one child that is TERRIBLE about tattling. He will do all sorts of mean and bullyish things to the other kids. He gets put in time out for it and is often made to play by himself for periods of time because of the way he treats people. He's also destructive and his mother and I have been working hard on training this out of him. AND YET...this child that is the king of sneaking will get out of his own time out to come and tattle on some other child for sneaking or even if he just thinks they are. He will tell in a hot second when another child is doing something, anything at all that he feels is against the rules. Sometimes it's not even really anything bad the child is doing. He just hopes to get them into trouble.
These are the kinds of things I talk a lot about with my kids. I think parents should talk about all kinds of scenarios with their children and the teachers at school should too.
I don't think it's really about teaching children that they should keep each others secrets out of solidarity. It's more an annoying thing for the grown ups. We get annoyed because it can take a big chunk out of our day if they start the constant tattling on each other. It's a game they play to try and get each other into trouble and we know it. So it annoys us. We need to be very CAREFUL. We don't want children to keep their mouths shut all the time. When they are in the older grades it could be about something serious like some girl is making herself throw up or someone else is threatning to bring a gun to school.
Sorry...I don't have an easy answer. TALK, TALK, and TALK some more with kids about this subject. It could be life or death.