My opinion is that they were just making it up to impress whoever they were telling. I would think that if they really did know a person who killed somebody, that the person wouldn't tell somebody who was likely to blab it in a school lunch room. I would guess that if someone was going to tell someone else a thing like that, it wouldn't be in a place where anyone could overhear.
If you had told the police, they would have made you tell who you heard say it and who they said it to. They would have gone to them and said someone told them they overheard it and question all of them, and they would deny it. If the kids figured out it was you who told the police, they would be the ones coming after you. Not to kill you or anything, I'm pretty sure, but to make the rest of your high school years horrible.
Again, they probably just made it up, because some kids are impressed by stuff like that. I don't think you should let it worry you. As for hearing them plot a murder, if anyone is stupid enough to plot a murder in a full lunch room at school, they will get caught. You don't even have to say anything. They will get caught.
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I'm giving it another shot. We will see.
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