...so you think that it should not be possible to charge people for choosing to not notice authorities if they have knowledge of a forthcoming case of mass murder? Because that's what you seem to be insinuating under the hyperbole and strawmen.
How do you prove that someone knew that someone else was planning an attack? If I told you, right here and right now, that I will be bombing my hometown's train station in exactly one hour, would you be compelled to contact the authorities, given all that you know about me through the interactions we've had online?
No, I wouldn't be. But it wouldn't exactly be a problem if I did and as a result the police caught you on your way out and took you in for questioning. Should you be charged? No. Should you be convicted? Definitely not. Is it wrong to stop you and check that you're not actually carrying a bomb and question you about your claimed intent? No.
Should I be charged for not reporting you? No. Should I be charged for not reporting you, if you actually did carry out the bombing? Seems understandable, even though I would disagree. Should I be convicted, in that case? No, because our interactions are on record and they suggest that it was entirely reasonable to not take your claim seriously.
However, if I believed that there's a decent chance that you're actually going to do it (for example, if you were someone I knew had attempted a bombing before) but still I didn't do anything, then yeah I should be convicted.
This is the same problem schools have when dealing with the possibility of a a spree killing by a current or former student. In those cases, there's usually a progression pointing towards the attacker contemplating and planning the attack days if not weeks before it happens, and yet despite all the previous cases, there are still ones that seem to happen out of the blue. Do you think that punishing the parents, teachers and friends of the killer(s) for not noticing what that person was planning would help prevent future killing sprees?
Obviously not. You'd have to prove that they actually knew or had a strong warranted suspicion and willfully turned a blind eye, just like you have to prove any allegation which involves whether the person knew or didn't know something. That's what courts do. If you don't have evidence that the person knew or clearly should have known, then of course they should not be punished. We don't decriminalize failing to provide assistance to a victim of an accident just because obviously it's possible that in some cases the person just genuinely didn't notice.
I have no doubt that in virtually all school shootings the parents, teachers and friends of the perpetrator didn't think that they'd actually do something like that. A plausible exception would be if they had told of their intentions to similarly-minded friends, who believed them but didn't try to stop it because they wanted it to happen too.
That's what I mean by punishing people for not being mindreaders. A sufficiently determined attacker will be able to conceal his or her intent until it's too late (This is analoguous to people who commit suicide despite being under psychiatric care); punishing the people around the attacker for not being able to figure out that person's true intentions is not going to help matters.
Now, a different case can be made about co-conspirators, people who were fully involved in planning the attacks and such, but as far as I can work out, those are already being prosecuted to the full extent of the law.
If there is a third category here, I would like to know what it is.
Criminals of all sorts reveal their intentions all the time out of stupidity, even though they could conceal them. Only smart and sufficiently determined ones won't.
A third category is of course people who aren't co-conspirators but are aware of what they are or have been up to.
If you want an easily palatable example, then consider a neo-nazi who their pals ask to help carry their ski masks and rifle-sized heavy bags into their unmarked van, while having heard them talk about good locations for a shooting spree and how they're totally gonna nail some of them n-words this time. Not a co-conspirator, yet (unless they're mentally handicapped) they should have known. If they get charged and convicted for failing to alert authorities, that's not punishing them for not being a mindreader.
Maybe there aren't any such non-co-conspirators who knew or should have known, maybe there are.