ばか！バカ！　馬鹿ー月！

The catch was this: I was fine with not paying attention in class, but it was almost impossible to not think of anything else, and even if I did, would life be happy that way? I don’t think Haruhi would want to lead that kind of boring, mundane life. As I started to gradually doubt her words, I suddenly recalled that her results were top notch, a huge contrast with reality.

What "reality?" The "huge contrast" could be with Kyon's reality of his own low grades, or with the predictions of his theory regarding Haruhi's inability to stay focused in class. It just cannot be that the reality of her top notch results are in contrast with...the reality of her top notch results.

I suspect the huge contrast is between the reality of Haruhi's grades and the attention deficit disorder diagnosis Kyon puts on Haruhi's behavior. If so, the fix could be to substitute "her erratic behavior" for "reality."

Er, what Kyon means is Haruhi told him to not think of anything else except the lesson. Kyon does so, finds it boring, and concludes that Haruhi is just pulling his leg since she wouldn't want that kind of boring life. But then he suddenly remembers that Haruhi's results are top-notch, and so laments about how contradicting Haruhi's methods are with her behavior, assuming that she really employed the said method in class.

My head hurts just trying to think about this one.

I think this is the relevant text from the Japanese version. Perhaps someone can offer a more informed opinion that I could:

ようは勉強しなくてもいいが勉強以外に頭を使うこともなーんもするなってわけだ。だが、そんな生活が楽しいわけがなく、実際にハルヒが何も考えていないことなんかあり得ないように思うので、ますます眉唾《まゆつば》であって、つまり全然当てにならないってもんだが、言ってるハルヒが好成績を維持《い　じ　》してのけている現実を突《つ》きつけられると一言もない

Either way, I think a substantial rewording may be required...
=Smidge=

Holycow's take on the meaning is similar to mine. The simple fix of subbing "her erratic behavior" for "reality" still leaves some of the meaning implicit, but the jump over the unstated is natural enough to my tastes. On the other hand, maybe a deeper revision would offer a prettier result. I'll leave it at that.