Why hasn't Jason skipped at least one grade? He's shown to be capable of the work of higher grades.
Imagine it's not just being academically talented, but also having a sufficient level of maturity... the latter which Jason clearly lacks.
Because, while "book smart", he has absolutely no common sense at all.
Age-Inappropriate Media?
Andy is well known for her strict nature concerning media and her children. She forbids them from playing violent video games (not that that stops Jason), and throws fits when they even think about seeing R-Rated movies. So why, then, does she allow her youngest son to read A Song of Ice and Fire, which is full of graphic sex and violence? Is she unaware of the book's content, thinking that all fantasy is like The Lord of the Rings, or does she simply think that books are harmless?
Given her reaction to some of Jason's video games ("Why isn't Donkey Kong wearing any pants?!"), it's entirely possible that Jason has kept the true nature of the series from her, much like when he convinced his dad the the "M" on video games stood for "Mild". She has demonstrated in the past that she has very few overlapping interests with Jason, so it's not likely that she would read the books and is probably unaware that A Game of Thrones is based on them, if she watches that at all.
Not just ASOIF. One strip implied that Peter and Paige rented Pulp Fiction and let Jason and Marcus watch it, and another had Jason working on his website when Marcus came in with a huge stack of South Park VHSs and asked Jason if he wanted them uploaded in any particular order. (Keep in mind that when Peter wanted to watch the same show on the MomVo, it told him "Fat chance. You're going to watch Nova".)
For that matter, it's interesting Jason likes a show partly known for its strong sexual content at all, given Jason's age (he's 10) and his traditional strongly negative attitude toward anything remotely "mushy." Such a show would seem better liked by (16-year-old/girl-obsessed) Peter.
Simple. A Song of Ice and Fire simply doesn't sound threatening and appears to involve music in some way
Ever had to read ostensibly child-friendly books in school or at least had them read to you by the teacher? Stuff like Where the Red Fern Grows and The Giver are often assigned to schoolkids despite being quite grim while the same teachers might take issue with any age-inappropriate movies/cartoons some of the students watch. The answer's simple: hypocrisy. Also, a video game, TV show or movie is easy to "catch" someone who shouldn't be near them because they can be displayed on a screen and make noise. Books don't and can more easily be ignored by adults.
He don't like video games?
Does it seem to anyone else that Amend doesn't really like video games, at least in the early years? During the debates between Andy and Jason, Andy's refusal to buy a new system is presented as the reasonable and mature sentiment (even at the end of the arc in which Jason wants a Super NES, in which Andy seems to think that the new system is just a ploy by Nintendo to sell more consoles), and the only times she comes off as hypocritical is when she actually plays the games themselves. When arguing with Andy about why she doesn't want to buy a system, Jason points out that it's too late to avoid him getting addicted to video games. At the end of the "Mortal Kombat" arc, Jason makes a speech about how childhood's too short to be wasted playing video games- he doesn't mean it, but the fact that he believes that it's a display of maturity that will enable him to get the game back seems to suggest that he believes this on some level. Even as late as the "Christmas Carol" parody arc, Marcus calls his clinging to video game controllers "selfish," even if that's not the advice he has to offer Jason. While the Foxes, especially Jason, often take their hobbies to excesses, this seems to b the only case in which Amend is commenting on the hobby itself.
Keep in mind that in the late '80's and early '90's, when the strip began, video games in America were being hit hard by a serious, often Federally-backed case of New Media Are Evil. You can read the gory details on the "Real Life" folder of that page, but a more "traditional" medium like a newspaper comic strip would generally have to follow the party line of "Even if they're enjoyable, they're intrinsically addictive and 'bad for you'". Later strips quietly dropped this along with the rest of the world. Even if Amend didn't really believe it, Andy almost certainly would. As for the "Christmas Carol" parody arc, you could read Marcusly's hoarding of controllers more as someone who wouldn't let their sibling or friend take a turn on a single-player game.
There're more vegetarian options,Andy.
Call me nuts, but as someone raised vegetarian, not all of Andy's cooking experiments sound bad like ginger pudding for dessert. Is it that Andy doesn't bother to improve at her cooking, and look up recipes online, or just her obsession with tofu?
I think you'll find most kids want nothing to do with tofu. I love it as an adult, but as a kid? Ewwww...
Why it took two years is a mystery, though.