Chad also states that he and Ryan walked in on Christopher masturbating.
Ryan's statement (THANKYOU for the links, btw) backs up Damon's, as far as the poo goes. Though I'm wondering atm if these were not two separate events? The cast who actually goes to the back of the school changes, as well as who did and saw what. Ryan did not witness the poo event according to himself and Damon, but is present for Chad's story. In Ryan's version there's a 'Smith' but not in the two other versions - in Ryan's they were also throwing the poo, but this isn't mentioned in the two others.
The three stories contain many of the same elements, though, enough to make me doubtful of two events. But hm. Dunno.
The Byers apparently had some concern over whether Christopher was being molested by somebody, due to some disturbing behaviour around that time (and that one of the many local pedos had been doing repairs for the Byers and paying too much attention to Chris..). I'd really like some direct quotes/links for that - Mark Byers discussed it in his book, but I am not purchasing that, so hopefully somebody has a copy and can tell us.