Personally, I don't even think motive plays into the equation here. By all accounts, KJ was very well liked at school. But let's say that was not true (however well documented it has been, both before and after his death).
For him to have been murdered in the manner suggested by some, so many things would have had to have happened in an impossibly small period of time, as well as a number of dark stars aligning:
-- first, multiple students would need to be involved. Whether he was rolled up in a mat which was then propped up, or lifted and dropped into a (let's say) 6' mat (itself, a difficult and risky task), one student could not have pulled it off.
-- the murderers would need to know that KJ would be going to the mat to retrieve his shoes, as they were found inside it, and they would have to have been laying in wait for him, never having been seen entering the gym and not leaving.
-- the murderers would have had to have killed him very quickly, with no mess or noise; KJ could not have struggled or have tried to escape.
-- If he was placed in the mat and then rolled up in it, the murderers would have then had to have unrolled a mat, placed him inside of it, rolled it up, and stood it on its end. The mat and KJ would have weighed hundreds of pounds.
-- the murderers would then have to disappear before any other students saw or heard them, evading the cameras at the gym's entrance and exit.
-- the murderers would have had to have ensured that they left no trace of their crime.
-- the murderers would have had to rely on a very wide-ranging conspiracy of silence, involving the school, coroner, SD, CID, and GBI. This would entail a huge number of bad people who care nothing about justice or human life.
I cannot fathom how anyone could see this as being more likely than a student reaching deeply into a mat to retrieve some shoes, getting stuck, and sinking down into it.