My thoughts have gone to somebody else throwing his shoes in there and he was retrieving them. No way even a young teen would think that was a good spot to store their own shoes and go through that several times a week. That makes absolutely no sense even for senseless teens.
That way the wording makes sense, too of "not enough evidence".
They just don't provide enough info about anything. The scenario you have is one I entertain as well. One of a few.
To me, if he was just in there for a class, was this gym class? It was early in the day so I assume so (can't recall)? Imo, you'd just leave them on a bleacher or out of the way to grab easily, why bother with putting them down inside there even if at that point it was an outside mat. If you are in the gym, they are in sight, you don't need to "hide" them.
What I don't know is they talk of to not "pay" for a locker... Is this a gym locker or a school locker? Because IF he was leaving them there all day, it makes more sense. Let's say he has practice after school. He doesn't want to put them in his school locker or perhaps he doesn't have one or take an extra pair of shoes to the next class. Then he wouldn't leave them on the bleachers or side of the gym in plain sight as he could during gym class. He'd want to "hide" them maybe and keep them out of the way as other people and classes would be in and out between his class and maybe practice later. THEN it makes more sense. But of course I've seen nothing that answers any of this in what I've looked at.
Here is the other problem and it is a big one. So mats were stored there over Christmas making that not an easily accessible mat. Well then why in January AFTER Christmas vacation would he use it in the first place if it were that far back and wasn't usually?? It was already blocked in... It makes NO SENSE. What does make sense is as you say someone threw them in there that far back INTENTIONALLY. At minimum.
Or did he leave them in it from before Christmas vacation and only first needed them again in mid January? They don't say, but I don't think so, that makes NO sense. Schools go back in session in most areas right after New Year's. And that then leads me to why were mats stored over Christmas break still there anyhow?
Nah. At minimum, SOMEONE threw his shoes in there. Scrambled back across the top of the mats to do it maybe... In fun taunting a friend or in meanness and bullying, etc. They probably may not have thought of him going in, just knew they created him a problem of moving a ton of mats to get his shoes or not being able to get them at all.
So I can entertain that scenario as you are and it isn't necessarily like murderously criminal although hiding it and not coming clean is "covering up" or hiding that action...
But then comes the problem of there weren't ANY shoes or evidence of shoes at the bottom of that rolled up mat that we have seen. And the problem becomes his going in head first and ending up like that when no shoes seem to have been there in the first place.
The shoes are, have been all along, and still are even after that synopsis a problem. As are other things.