Summarize this article in one sentence.
The teacher may have seen the note but would not have realized it and would've thought that a random student who went to throw away a piece a paper just felt nice enough to pick up someone else's trash.

Summary:
Start with a small piece of paper such as an index card and rip it as much as you can without ripping it into multiple pieces  Write your note naturally and then fold your index card with the torn parts so it's about a 1x1 inch square with folds on the edge so it looks like a bit of ripped paper going to the trash. Walk up the the trash and throw it close to the trash can but not in. Have your friend afterwards walk up the trash can, pick up the note as if it was just a random piece of paper and when they get up, switch the note with any piece of paper they have that looks like the note which they would throw into the trash. Done!