This has similarities to the Sanjay and Craig origin. So don't say, "Hurr hurr, you copied Sanjay and Craig's origin!" Yes, I did, but it was my imagination! No mean comments, please....
If you watch Clarence, then you'll notice very out-of-place things in the background.
Let me explain.
In some episodes, in the background you'll notice odd creatures or a pair of eyes looking under the bushes. This is true, next time you watch Clarence, just look very carefully in the background, you'll see it.
This explains a very strange theory, and all of the characters are dead, the show is just the past of the friends, the whole theory is down below.
Clarence has huge, mental, problems in the show. That's why he's funny, and hyper. Clarence died in a car crash when Clarence's mom brought Clarence to school. When the car crashed, it crashed into a concrete wall causing Clarence to have a huge nose bleed. At least, his mom survived. But, she had serious brain injuries, and had mental problems, leading her to suicide.
Sumo was made fun of in class at all times, because of his hair, and his funny voice. People called him, "Loser." Sumo got angry at this, but he knew that there was nothing he could do. It got worse over the years, and Sumo became pushed around, until his father beat him to death because he was not standing up for himself.
Jeff is insane for tests. Selfishness and greediness took over him. When he was in Ms. Shoop's class, it was causing him to go insane, and freak out. He was seething with rage. He destroyed Ms. Baker's class. Then, he told Ms. Baker to get back into her class. He continues jumping, and destroying everything in the room. He jumped to high and he hit the ceiling, and was decaipited.
This is what lies behind "Clarence" and it's sad, but it's the truth. They're all dead from different accidents, and went the place that dead souls go, another world. Souls go there when their body dies, and Clarence, Sumo, and Jeff went there.
The whole series depicts that place, and in that place they are always being watched by gaurdians so they don't escape and try to live in another soul. This explains the weird creatures in the background, watching Clarence while they do things with their dead friends.