Spoilers
There are two big theories that I've seen.
1. The first is that the author is TJ. This theory posits that TJ feels guilt over what happened to Sydney and had been creating the notes and hiding them on his own. He is the one to know where all notes are located, examples of this being that it was mentioned that he can move silently through the old school house, and knew where the note was hidden in Sydney's old room. The main issue with the theory is that TJ is not the type to break and enter, commit crimes to hide these notes, and seems to not know what the notes are going to say at times.
2. The other theory is that Sydney is possessing TJ and influencing him to create and hide the notes. Either way TJ is physically hiding the notes, but his is pushed by Sydney to do so. This would also explain the antagonistic aspects of the notes regarding Chase, since TJ does like Chase and wouldn't likely be pointing out Chase killed Sydney, since he did so to protect TJ.
Neither theory explains everything, but most likely TJ was somehow involved.