The kids appear older than they are. For Arthur it's ok, since he kind of is, but the rest of the bunch act nothing like 6th graders do. If I were fed no info about their age, I would guess 8th grade, minimum.Two of the important characters, Danny and Mike, are in the eighth grade.
To the reviewer, if he reads this, thank you for the kind review. It's just that one part that struck me as strange. Has the spelling been getting any better in the later chapters?