I would simply have told him that he misspelled myth.

Perhaps the youngster is just brilliant. I found one meaning of “mith” that certainly applies...

*** A fiery, scary God.

I would be very upset to find that Christian kids were allowed to post scriptures and notes promoting Christianity around school. Based on that I would feel obligated to take a similar position against posting pro agnostic notes in spite of my person support for it. However it is very hard to believe that a second grader would do that without significant support to do so and doubt the truthfulness of the post.
BTW: I don't give a shit how you spell myth and find the criticism arrogant.

Befriend the parents?

But really... in 2nd grade during a troop camping my daughter did the same verbally. Not sure how it started.. but I walked into her saying "We are all made of stars..." and girls replying "no! God made us!!".

We have a secular troop. So we explained that we respect everyone's beliefs. Everyone said what their religion or belief was... we had Pentecostal, Catholic, Jewish, non-denominational Christian, some didn't know...only that God made them. In the end they concluded that the reason we are secular was a choice made by the adult leaders. We were on our 3rd year completion together and our different beliefs had no impact on our dynamic and we would continue being Girl Scout sisters even after we found out our differences. We discussed freedom of religion and asked them how to apply the Girl Scout law to what we just learned about eachother.

I think it went well. We are on our 5th year now ?

Not sure if I believe this actually happened, but whatever.
I wouldn't have "handled things so that everybody was pacified".
I would have corrected the misspelling of 'myth'.
I would have made the little post-it poster clean up his mess.
I would have told the ones who had problems to talk to their parents.

Chastise the kid for littering. Tell the other kids that everyone has a right to his or her own opinion and that they are not to harass the kid for his.

Just sit back and let it play out. That kid is going places.

I would have told the other kids that he is entitled to his views and to believe or not as he chooses. In private, I would have told him that people are very sensitive to having their beliefs challenged, it might be best to keep that particular belief to himself, and god is a myth M-Y-T-H.

I'd give little Timmy or Tanya a juice box, correct their grammatical error, and give them an A for being able to see through the wall of fear mongering and deceit.

Then of course I would wait to be escorted off the premises by the school resource officer since I'm not a teacher at the school, I don't have any children that attend the school, and I generally have no business being there. ?

Tell all the kids that people hold different beliefs. And leave it at that.

tell him/her to spell better and give them the atheists secret handshake

Give him detention for his appalling spelling.

I'd guide a discussion with the kids. I'd ask them questions and give them a chance to think about what exactly they are upset about and why. I'd emphasize that different people believe different things and that's okay but it's not okay to be disrespectful.

Noticed many mentioned spelling. Some schools teach phonetically writing and work on spelling later (as a teacher of teens who went through that, I wasn't a fan). Easiest way to avoid trouble, tell him you understand and he has a right to his opinion. Then tell him how non environmentally friendly out door post it notes are.

You teach them all how to spell myth and then tell them to go home and ask their parents about god

A good way to bring up some 1st amendment issues. Freedom of expression, and debate about a person's right to express a belief. "I mean how is this different from a person writing a note that says Jesus loves you and would that be acceptable. Btw, spelling check might help also.

I'd explain people have different believes, in America it's a right and guaranteed by the Constitution.
But placing stickers on school equipment is not permitted. Keeping it factual and non judgmental. THEN I would address the correct spelling of myth.

I would have him help you take them down and tell him not to do that for littering reasons.

Discretion is the grander part of valour. He should absolutely know that he has the right to his beliefs and that others have a right to theirs. Just remember that you are a unique individual, just like everybody else. Weaponizing differences will not make him many friends.

I"d have given him or her a hug and helped clean them up. But would have mentioned in passing that I agree.

He is merely supporting the separation of church and state. =]

I hope you give them an A+

My first thought: how much more angry would parents have been if he had posted "Santa isn't real"

Public school, correct the spelling?
Then maybe speak of the many other ancient mythical creatures that abound.
Please tread carefully sounds worse than transporting nitro glycerin.?

do not believe you, what kid does that ?